Preheat the oven to 350°F.

Melt ½ cup unsalted butter (1 stick) in a large skillet over medium-high heat.

While the butter melts, finely chop 1 medium onion and 1 celery rib, slice 2 tablespoons green onions and reserve some of the green parts for garnish, and mince 1 clove garlic.

Add the chopped onion, chopped celery, the white/light-green parts of the sliced green onions, and the minced garlic to the skillet. Cook, stirring, until the vegetables are softened, about 5 minutes.

While the vegetables cook, vigorously whisk together 2 large egg yolks (slightly beaten) and the contents of 1 (12-oz.) can evaporated milk in a small bowl until well blended; set aside.

Sprinkle 2 tablespoons all-purpose flour over the vegetables and stir to blend thoroughly, cooking about 2 minutes to form a white roux. Do not allow the flour to brown.

Using a whisk, add the egg-and-milk mixture to the skillet slowly in a steady stream, whisking constantly to incorporate and prevent curdling.

Stir in the juice of one lemon, 1 teaspoon hot sauce, 1 teaspoon Kosher salt, ½ teaspoon cayenne pepper, and ½ teaspoon freshly ground black pepper. Continue to cook and stir for 4–5 minutes, until the sauce thickens to the consistency of a thick creamed soup.

Remove the skillet from the heat and fold in half of the grated cheese (use half of the Gruyere and half of the Fontina) until fully melted and incorporated.

Pick over 1 lb lump crab meat for shells and cartilage, then gently divide the crab among four mini ovenproof casseroles (or place in one large cast-iron skillet or an 8"x8" baking dish), taking care not to break up the lump crab.

Pour the cheese sauce evenly over the crab, then sprinkle the remaining grated Gruyere and Fontina on top.

Bake at 350°F until bubbly, about 15 minutes. Switch the oven to broil and broil until the cheese begins to brown, about 3–5 minutes—watch closely to avoid burning.

Remove from the oven, sprinkle with the reserved green parts of the sliced green onions, and serve immediately.