Preheat oven to 350 degrees F.
Prepare Kraft Dinner as per package instructions.
While pasta cooks, in a large frying pan or Dutch oven, melt butter on medium heat. Saute onions and carrots for about 5 minutes.
Add the roast beef and drippings and saute for another 5-10 minutes until carrots are almost done.
Add frozen peas and spinach and cook for another 5 minutes until spinach begins to wilt. Do not overcook at this point.
Salt and pepper to taste and remove from heat.
Add prepared Kraft Dinner and 1 cup of cheddar cheese to the mix. Stir to combine.
Transfer to a greased 9 by 13 inch casserole dish, or leave everything in the Dutch oven. Sprinkle on remaining cheddar and Monterey jack cheese.
Bake, uncovered, at 350 degrees F for 20 minutes.