Preheat oven to 375 degrees.
Spray a 9x13 casserole dish lightly with cooking spray. Spread about 1/4 cup of the refried beans on the bottom of the dish to help the taco shells "stick" and stand up.
Spread the remaining refried beans evenly along the bottom of the taco shells and arrange the filled shells standing in the casserole dish.
Heat a non-stick skillet over medium-high heat. Add the onion and cook for a couple of minutes. Add the ground sirloin, breaking up with a wooden spoon or spatula. Add the cumin and salt. Cook until the ground sirloin is no longer pink.
Drain the Ro*Tel tomatoes and add half of them to the cooked ground beef mixture. Stir to combine.
Reduce heat to medium low and add the cubed Velveeta cheese. Stir and heat until completely melted.
Scoop the cheesy beef filling evenly into each of the bean-filled taco shells.
Top each taco with shredded Monterey Jack cheese.
Bake the tacos for 15 minutes.
Remove from oven and top with lettuce, the remaining Ro*Tel tomatoes and sour cream. The tacos are best served in a bowl with a fork to eat them!