Preheat the oven to 375°F (190°C).
Cook the jumbo pasta shells according to package directions until al dente; drain, spread on a large plate, and let cool slightly so they are easy to handle.
Heat the olive oil in a large saucepan over medium heat, add the minced garlic and sauté for about 2 minutes until fragrant.
Add the ground beef to the pan and cook, breaking it up, until no longer pink.
Stir in the taco seasoning, salt, and black pepper, then add 1/4 cup water and simmer for 5 minutes.
Fold in the drained black beans and cook for 2 more minutes; remove from heat and let the meat mixture cool slightly.
Mix the crushed tomatoes and salsa together, then spread three-quarters of this sauce over the bottom of a 13x9-inch casserole dish.
Fill each cooled pasta shell with the meat and bean mixture and arrange the filled shells in the prepared dish over the sauce.
Spoon the remaining sauce over the arranged shells, sprinkle the shredded cheddar and Monterey Jack cheese evenly on top, and bake for 18 minutes or until bubbly and the cheese is melted.
Remove from the oven and top with chopped Roma tomatoes and chopped green onions before serving.