Preheat the oven to 400°. In a large baking dish or ovenproof skillet, mix the cheeses. Bake until brown and bubbly in spots, about 20 minutes.
In medium skillet, cook 1/3 cup each chopped onion, red bell pepper and green bell pepper in 1 tbsp. oil over medium heat until starting to soften, about 3 minutes. Add 1 cup thinly sliced top-sirloin steak; cook until browned, about 2 minutes. Add to the queso, then get dipping!