Preheat oven to 400 degrees; line a baking sheet with parchment paper.
Whisk onion, half and half, cornstarch, egg, salt, and spices together in large bowl.
Add ground meats, blend to incorporatae, then scoop into thirty 1 1/2" meatballs, and place on the prepared baking sheet.
Heat butter and oil in a nonstick skillet over medium heat.
Roll meatballs between your palms, then brown in the butter mixture (in batches), about 2 minutes per side; do not cook through. Carefully transfer meatballs to a dutch oven, pour off fat from skillet, and return it to the burner over medium heat.
Prepare brown gravy:.
Deglaze the skillet the meatballs were sauteed in with 3 1/2 cups broth, scraping brown bits from the bottom of the pan. Stir in beef base and bring to a boil over medium heat.
Meanwhile, shisk flour and kitchen bouquet ina cut with the remaining 1/2 cup broth. Add this mixture to the simmering broth, stirring constantly.
Cook 5 minutes, skimming if necessary, then season with salt and pepper. Pour gravy over meatballs, cover pot, and braise 35-40 minutes in oven, or until meatballs are cooked through. Serve over mashed potoes or buttered noodles.