Preheat the oven to 375°F (190°C). Cook jumbo pasta shells according to package directions until al dente, drain, spread on a large plate, and let cool slightly so they can be filled.
Heat 1 tablespoon olive oil in a large saucepan over medium heat. Add the minced garlic and sauté about 1–2 minutes until fragrant, stirring frequently.
Add the ground beef to the pan and cook until browned and no longer pink, breaking it up with a spoon as it cooks.
Stir in the taco seasoning, 1/2 teaspoon salt, and 1/4 teaspoon black pepper, then add 1/4 cup water and simmer for about 5 minutes.
Stir in the drained and rinsed black beans and cook 1–2 minutes more to heat through, then remove the pan from the heat and let the filling cool slightly.
In a small bowl, combine 1 1/4 cups crushed tomatoes and 3/4 cup salsa. Spread about three quarters of this sauce over the bottom of a 13x9-inch (or similar) casserole dish.
Spoon the beef and bean mixture into each cooled pasta shell and arrange the filled shells in the prepared baking dish over the sauce.
Pour the remaining sauce over the arranged shells, then sprinkle 1 1/2 cups shredded cheddar and Monterey Jack cheese evenly on top.
Bake in the preheated oven for 18 minutes, or until the casserole is bubbly and the cheese is melted.
Remove from the oven and top with the chopped Roma tomatoes and chopped green onions before serving.
