Heat oven to 425°F. Arrange baguette slices on a sheet pan and brush with olive oil.
Toast bread until golden, 6–8 minutes. Set aside and switch oven to broil.
Warm marinara in a large skillet over medium heat. Stir in garlic, Italian seasoning, and red pepper flakes.
Add meatballs and toss to coat. Cover and simmer until heated through, 8–10 minutes.
Spoon a little sauce onto each toast. Top with one meatball and more sauce.
Sprinkle with mozzarella and Parmesan. Broil until cheese melts and bubbles, 1–2 minutes.
Shower with basil and season with salt and pepper. Serve warm.