Place ground beef in a large bowl. Add the minced onion, egg and bread crumbs. Season well with sea salt and pepper. Form the mixture into meatballs the size of ping pong balls.
Heat 1/4-inch of oil in a skillet over medium heat. Fry the meatballs until they are browned on all sides and cooked through, about 15 minutes. Remove meatballs from pan. Add sliced onion and sauté until soft and translucent.
Bring a large pot of heavily salted water to a boil over medium-high heat.
While the water is heating, add peas to the pan. Cook until they are tender (but not too soft or mushy), 3 to 4 minutes. Stir in garlic and cook 1 minute more.
Season with sea salt and pepper; add the meatballs back in the pan.
Drop pasta into boiling water and cook until al dente. Drain well and transfer to a large bowl. Immediately toss pasta with peas, meatballs, crème fraîche and basil. Season with sea salt and pepper, to taste. Add more crème fraîche and basil, if desired.