Preheat oven to 400 degrees.In a large skillet, cook and crumble the sausage until brown. Remove from skillet and drain on a paper towel. Pour off any excess fat. Add onions and bell peppers to the skillet and cook over medium-high heat until the veggies are soft and golden brown, about 7-8 minutes. Remove from heat and set aside. To build the skillet, place 1/3 of the cheese in a medium-sized ovenproof skillet. Add half the sausage. Add another third of the cheese, then as much of the veggie mixture as you'd like (you may have a little left over.) Add almost all of the remaining cheese, the rest of the sausage, then whatever cheese you have left. Sprinkle the top very lightly with chili powder.NOTE: Depending on the size of your skillet or baking dish, you may have some ingredients left over. Just build so that the ingredients roughly reach the top of the skillet; they'll shrink quite a bit once the cheese melts!Place into the oven and start watching it about 4 to 5 minutes in. You want to bake it until the cheese is totally melted, hot, and slightly bubbling...but before the cheese starts to firm up/harden. (In other words, you still want the cheese to be extra gooey.)Remove from the oven, top with the diced tomatoes, sprinkle on the tomatoes, and serve immediately with tortilla chips!NOTE: Be sure to have everything ready so you can serve the queso fundido pretty much right out of the oven. It's best when piping hot! IMPORTANT: Wrap a cloth around the skillet handle so guests won't burn themselves.