Slice each chicken breast lengthwise to make 1/2-inch-thick cutlets.
Whisk the egg with 1 tablespoon water in a shallow bowl until combined.
In another shallow bowl or plate, combine the almond flour, grated Parmesan, garlic powder, and sea salt; mix well.
Preheat the oven to 425°F (220°C).
Dredge each cutlet first in the egg mixture, shaking off excess, then press into the almond flour mixture to coat both sides thoroughly.
Heat 2 to 3 tablespoons avocado oil in a large non-stick skillet over medium-high heat.
Pan-fry the cutlets 2 to 3 minutes per side, until golden brown on both sides and almost cooked through.
Place the seared cutlets in a casserole or baking dish in a single layer.
Top each cutlet with slices of fresh mozzarella, then pour the marinara sauce over and around the chicken.
Bake in the preheated oven for about 15 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and the cheese is melted.
Remove from the oven and serve immediately.