6 medium zucchini (about 3 lb.)
3 tbsp. olive oil, divided
1 lb. mild or spicy Italian sausage
1 small red bell pepper, diced
2 cloves garlic, minced
1/4 tsp. crushed red pepper flakes (optional)
1/2 tsp. kosher salt
1 tsp. chopped fresh oregano, plus more for serving
2 tbsp. chopped fresh basil, plus more for serving
2 c. marinara sauce
1 c. shredded mozzarella cheese
1/2 c. grated parmesan cheese, divided
1/2 c. panko breadcrumbs