Preheat your oven to 400 degrees f.
Heat a large skillet over medium-high heat.
Add in ground meat, garlic, and cook while crumbling with a wooden spoon until no longer pink, about 6-8 minutes.
Drain excess grease.
Season with Italian seasoning, sea salt and pepper, then stir in the diced tomatoes.
Continue to simmer on medium-low heat for 8-10 minutes, or until sauce thickens a bit.
Pour your marinara sauce into the bottom of the casserole dish and spread with a spatula.
Arrange the peppers, cut side up as shown, and evenly spoon the meat mixture inside.
Sprinkle with shredded cheese and arrange pepperoni slices and olives on top.
Bake for about 15-20 minutes or so until peppers are tender-crisp and cheese is nice and melty.
Sprinkle with red pepper flakes if desired.