Preheat the oven to 350˚F. Cook veggies and pour into a greased casserole dish (I used a deep 9-inch pie pan).
While the vegetables are cooking, make the cream sauce. Whisk butter, flour, and diced onion while cooking it over medium-low heat until it is light brown, for about 3 minutes. Add milk, salt and pepper continue whisking over medium-low heat until thickened. Add cream cheese and stir until smooth.
Pour cream sauce over veggies. Gently mix until the veggies are coated.
Sprinkle with cheddar cheese and bake at 350˚F for 25 minutes.
Sprinkle French fried onions on top and bake another 5 minutes.