Preheat slow cooker to High.

Clean neck bones and remove any excess fat. Season bones with cumin, allspice, black pepper, red pepper flakes, and salt; dust with 1/4 cup flour.

Heat olive oil in a large skillet over medium heat; cook and stir garlic until fragrant, 1 to 2 minutes. Increase heat to medium-high and add seasoned neck bones; cook until bones begin to brown, about 1 minute per side.

Transfer neck bones and drippings to the preheated slow cooker; add beef broth and onion.

Cook on High for 1 hour. Reduce heat to Low and cook 4 to 6 hours.

Transfer neck bones to a dish. Sprinkle remaining flour into the slow cooker and stir well. Return neck bones to slow cooker. Let stand until gravy thickens, 15 to 20 minutes.