Sloppy Joe Casserole

Name: Sloppy Joe Casserole
Ingredients: 1 large yellow onion (about 10 ounces); 2 small red bell peppers (about 6 ounces each); 6 large cloves garlic, minced; 4 tablespoons olive oil, divided; 1 3/4 teaspoons kosher salt, divided; 2 1/2 pounds lean ground beef; 2 (15-ounce) cans tomato sauce; 3/4 cup ketchup; 2 tablespoons apple cider vinegar; 2 tablespoons packed light brown sugar; 2 teaspoons prepared yellow mustard; 2 teaspoons Worcestershire sauce; 8 to 12 ounces shredded Mexican blend cheese (2 to 3 cups); 12 slider buns or soft dinner rolls; 4 tablespoons (1/2 stick) unsalted butter; 1 tablespoons poppy seeds