Bring a large pot of water to a rolling boil over high heat. Drop the trimmed french green beans into the water and boil them for exactly 4 minutes. Prepare an ice bath in a large clear glass mixing bowl while they cook. Transfer the green beans immediately into the ice water using tongs to stop the cooking process. The green beans will turn a , bright green color and feel slightly tender but still crisp. You'll smell the fresh scent of the cooked greens right away. (I tried using canned green beans once - the whole dish turned into a sad mushy pile. Fresh beans are non-negotiable here.) Miss this window and the greens turn a dull khaki color and get watery. Plunge them into ice water immediately - the rapid temperature drop stops the cooking so the beans keep their snappy structure.

Heat the extra virgin olive oil in a heavy 10-inch round cast-iron skillet over medium-high heat until hot but not smoking. Add the sliced cremini mushrooms and diced yellow onion. Cook for 6 to 8 minutes while stirring occasionally with a wooden spoon. Stir in the minced garlic cloves during the last minute of cooking. The mushrooms will release their liquid and shrink down while turning a deep chestnut brown color. You will hear a loud sizzling sound that slows down as the moisture evaporates from the pan. The onion pieces will become translucent with lightly browned edges. Go past 8 minutes and the garlic will scorch and taste bitter. Cook until the moisture evaporates completely - removing the water concentrates the rich savory flavor of the mushrooms.

Sprinkle the unbleached all-purpose flour over the sautéed mushrooms in the skillet. Stir constantly for 60 seconds over medium heat to coat the vegetables. Slowly pour in the organic heavy cream and lower the heat to medium-low. Simmer the mixture for 4 to 5 minutes while scraping the bottom of the pan with a spatula. The pale cream blends with the brown pan drippings to form a glossy, thick beige sauce. The mixture will bubble gently around the edges of the skillet. The sauce coats the back of a spoon and a finger drawn across leaves a clean line. Miss this and the raw flour taste stays inside the sauce and ruins the creaminess. Simmer gently until thick - the slow heat activates the starches without curdling the heavy cream.

Remove the skillet from the heat entirely. Add the drained french green beans, half of the crispy fried onions, and the shredded sharp cheddar cheese directly into the pan. Fold gently with a rubber spatula in a J-shaped motion until everything is evenly combined. The bright green beans get coated in the glossy beige sauce with flecks of melted orange cheese visible throughout. The fried onions soften slightly into the mixture. The entire combination looks thick and holds its shape loosely when piled in the pan. Skip the gentle fold and the delicate green beans break and release excess moisture. Mix off the heat completely - keeping the pan cool prevents the cheese from separating into a greasy film.

Transfer the mixture into a 9x13 white ceramic baking dish and spread it into an uneven layer. Bake at 375°F in a preheated oven for 20 minutes. Remove the dish and scatter the remaining crispy fried onions across the top. Bake for another 5 minutes until the onion strings turn deep golden brown. The sauce bubbles fiercely around the edges of the white dish and turns a light amber color. The top layer of onion strings becomes incredibly crunchy and dark golden. You will smell a rich, sweet toasted onion aroma filling your kitchen. Go past 5 minutes on the final bake and the delicate onion strings burn and turn black. Bake at 375°F instead of 400°F - the moderate heat prevents the outer onions from burning before the center warms through.

Let the casserole rest for 10 minutes off the heat before serving. Scoop a generous single portion using a large serving spoon and place it onto a clean white plate. The portion should show an uneven texture with layers of creamy beans and a thick crunchy topping. The creamy beige sauce pools slightly at the base of the pile without running across the plate. The green beans look plump and under the dark golden onion crust. A single stray onion string rests casually to the side of the main pile. Skip the rest and the juices run out the moment you scoop your first portion. Rest 10 minutes before serving - gives the cream sauce time to set so it stays together cleanly.