Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Finely mince the portobello mushroom, celery stalk, and garlic clove into very small, uniform pieces.
Heat the extra virgin olive oil in a non-stick skillet over medium heat and sauté the minced mushroom, celery, and garlic until they are softened and fragrant.
In a large mixing bowl, whisk the egg until it becomes slightly frothy and well combined.
Add the cooled sautéed vegetable mixture and the ground beef to the bowl with the egg.
Season the mixture with the sea salt, cumin powder, paprika, and black pepper.
Gently mix the ingredients together with your hands until just combined, taking care not to overwork the meat to keep the texture light.
Roll the mixture into 1.5-inch balls and arrange them evenly on the prepared baking sheet.
Bake for 15 to 18 minutes or until the meatballs are browned on the outside and fully cooked through.