Pulse half of the onion, garlic, sage, fennel seeds, coriander, cumin and 1 Tbsp. salt in a food processor, scraping down sides as needed until finely chopped. Transfer to a bowl and mix together with the egg and lamb. Form mixture into 1 ½" balls.
Heat 2 Tbsp. oil in a large skillet, over medium-high flame. Cook meatballs, turning occasionally, until browned on all sides and cooked through, 5–8 minutes. Remove meatballs from skillet and add the rest of the onions. Sauté until brown, then add coconut milk and peas. Bring to a low simmer and cook for for 5 minutes. Season with salt to taste. Return meatballs to pan and cook for one minute. Sprinkle with some minced sage and serve.