Combine the ground chicken, egg, almond flour, Parmesan, Italian seasoning, garlic powder, salt, and pepper in a large bowl. Mix gently, just until combined; overmixing can make the meatballs dense.

With lightly oiled or damp hands, form 16 even meatballs, about 1 1/2 inches wide. Keep the raw chicken mixture chilled if you are not cooking immediately.

Heat the olive oil in a 12-inch skillet over medium heat. Brown the meatballs for about 2 to 3 minutes per side, turning carefully. They do not need to be cooked through yet. Transfer to a clean plate.

Reduce heat to medium-low and melt the butter in the same pan. Stir in the garlic and cook for 30 seconds until fragrant, without browning it.

Add chicken broth and scrape up the flavorful browned bits. Whisk in Dijon mustard and heavy cream. Bring to a gentle simmer; avoid a rolling boil.

Return the meatballs and any accumulated juices to the skillet. Simmer gently, partially covered, for 8 to 12 minutes, turning once, until a thermometer inserted into the center of the largest meatball reads 165°F (74°C). If the sauce gets too thick, stir in a splash of broth.

Fold the spinach into the sauce and cook for 1 to 2 minutes until wilted. Remove from the heat, add the lemon juice, taste, and adjust seasoning.

Divide the 16 meatballs among four plates, spoon over the spinach-Dijon sauce, and garnish with parsley. Serve immediately with cauliflower rice, sautéed zucchini, or a side salad.