Prepare the sauce by combining garlic, soy sauce, hoisin, sesame oil, and red pepper flakes in a small bowl; set aside.
Bring a large pot of water to a boil, and add udon blocks. Let them sit in the water until they start to unwind, 1 to 2 minutes. Noodles should remain a little chewy. Drain noodles, and return to the pot.
Add sauce, and stir until noodles are well coated. Move to a serving dish, garnish with sliced green onions and sesame seeds, and serve.
