I wind up making this slightly different each time, but apparently if you follow the recipe as I wrote it down for mom it turns out great. The trick to tender meatballs of any kind is to add pureed onion (use a food processor or blender). Using a mixture of meats helps too. I often cook mine in the pressure cooker for extra tenderness, but that's optional.
1/2 lb. ground beef
1/2 lb. ground pork
1 small or 1/2 large onion, pureed
1/4 c. matzo meal or breadcrumbs
1 egg yolk
1/4 c. parsley, minced
2-3 cloves garlic, minced
1 Tbsp. whole fennel seeds
1 Tbsp. salt
2 tsp. black pepper
1 tsp. oregano
~1 tsp. red pepper flakes
~ 1/4 c. oil for frying
2 Tbsp. tomato paste
1/2 c. boiling water or broth
1/4 c. red wine
Combine all the ingredients thoroughly and form into 1-1.5" balls. Fry the meatballs in a generous quantity of oil in a single layer. Turn them carefully so that they are seared brown on as many sides as you have patience to fry (turn them at least once).
Mix the tomato paste, boiling water, and red wine. Pour this liquid into the pan so that the bottom of the pan is covered and 1/4-1/2" of the meatballs is submerged (add more water or wine if you need to, depending on the size of your pan). Cover and simmer the meatballs for 15-20 minutes or pressure cook for 12 minutes, making sure that the liquid doesn't evaporate completely.
Make the rest of your sauce however you were planning. Toss with spaghetti and enjoy!
The spices are merely guidelines. As long as you have pureed onion, fennel seeds, and red pepper flakes, you're on the right track. Experiment with using fresh basil instead of (or with) the parsley, or adding thyme or marjoram. Some people add parmesan, but I think that makes a mess in the pan.