Preheat the oven: Set it to 400°F (200°C). Lightly oil a 9x13-inch baking dish or large oven-safe skillet.
Mix the meatball base: In a large bowl, combine ground beef, Italian sausage, eggs, almond flour (or pork rinds), Parmesan, onion, garlic, parsley, Italian seasoning, red pepper flakes, salt, and pepper.
Use your hands to gently mix until just combined. Don’t overwork the meat.
Shape the meatballs: Roll into golf-ball-sized meatballs (about 1 1/2 inches). You should get around 18–24, depending on size.
Par-bake for structure: Arrange meatballs in the dish in a single layer.
Bake for 12–15 minutes until they start to firm up and release some juices.
Add the sauce: Remove the dish from the oven and pour marinara over and around the meatballs. Spoon a bit on top of each one to keep them moist.
Finish baking: Return to the oven for 10 minutes to let the flavors meld and the sauce heat through.
Add cheese: Sprinkle mozzarella evenly over the top. Bake another 8–10 minutes, until the cheese is melted and bubbly.
For golden spots, broil for 1–2 minutes.
Rest and garnish: Let it rest 5 minutes so the sauce settles. Drizzle with a little olive oil and scatter fresh basil over the top.
Serve: Enjoy as-is, or pair with zucchini noodles, roasted broccoli, spaghetti squash, or a crisp salad.