Prepare the Noodles: Cook udon or soba noodles according to package instructions; drain and set aside.

Make the Sauce: Combine soy sauce, mirin, sugar, and a touch of sesame oil for a balanced sauce.

Sauté Vegetables: Stir-fry onions, bell peppers, and mushrooms in a hot pan with oil.

Add Noodles: Toss cooked noodles into the pan with the sautéed vegetables.

Pour the Sauce: Add the prepared sauce to the pan, coating the noodles evenly.

Add Protein (Optional): Include tofu, chicken, or shrimp for extra protein and texture.

Garnish: Top with sliced green onions, sesame seeds, and a sprinkle of red pepper flakes.

Serve Hot: Plate the noodles and serve immediately for the best flavor.