In a large saute pan, fry the bacon until half-cooked. Add the mushrooms and red onions and saute until the bacon is fully cooked and onions have softened. Increase the heat to high. Add the collard greens and cook until the greens are wilted. Turn off the heat and set the pan aside.
In a separate pan, melt the butter and then add the flour. Cook, whisking frequently, to create a smooth, blond roux (this will take 3 to 4 minutes; do not brown, as the goal is just to remove the raw taste from the flour). Slowly add the cream while whisking and adjust the heat to keep the sauce at a simmer. Add the salt and pepper to taste (it should taste like a moderately salty cream gravy). Continue cooking until the sauce thickens and easily coats the back of a spoon.
Butter an 8-by-12-inch casserole dish (or 8-to-10-cup capacity casserole). Combine the collard greens with cream sauce and transfer to the casserole. Top with the cheese and french-fried onions. Bake, uncovered, until the cheese begins to brown, 15 to 20 minutes.
This recipe was provided by a chef, restaurant or culinary professional and may have been scaled down from a bulk recipe. The Food Network Kitchens chefs have not tested this recipe, in the proportions indicated, and therefore, we cannot make any representation as to the results.