Preheat your oven to 450 degrees F. Line and grease a large sheet tray with aluminum foil and oil. Melt butter in a medium saucepan on low heat. Finely dice onions and add to saucepan. Increase heat to medium and stir with a spatula for 1-2 minutes. Add garlic and thyme. Stir until butter is mostly evaporated and absorbed by onions, about 5 minutes.
Transfer ¾ of the sauteed onions to a large mixing bowl. Keep the remaining ¼ onions in the saucepan for the base of the sauce. In the large mixing bowl, add all remaining meatball ingredients except for ground meat: egg, panko breadcrumbs, parsley, olive oil, salt, ground black pepper, and chili flakes.
Add 1 Tbsp of the cranberry jelly to the meatball mix and empty the rest of the jelly, and glaze ingredients into the saucepan. Return saucepan to medium heat and whisk until the glaze begins to simmer (bubbles form and pop around the pot). Set glaze aside.
Stir all ingredients in the mixing bowl until fully combined and breadcrumbs are hydrated. Add ground beef into the meatball mix and toss with one hand until the breadcrumb mix is fully incorporated with the meat. Divide and shape meatball mix into about 18-20 one inch meatballs and arrange on your lined sheet pan. Bake meatballs for 9 minutes.
Remove meatballs from the oven and use a pastry brush to spread glaze over the meatballs. Return to oven for another 2 minutes. Remove glazed meatballs from the oven and serve with more glaze and olives for garnish.