Spicy Cashew Chicken over Udon Noodles

Name: Spicy Cashew Chicken over Udon Noodles
Ingredients: 6 whole Boneless Skinless Chicken Thighs, Cubed; 3 cloves Garlic, Minced; 1-½ Tablespoon Soy Sauce, Divided; 2 teaspoons Fish Sauce, Divided; 2-¼ teaspoons Chili Pepper Flakes, Divided; 1-½ teaspoon Sesame Oil, Divided; 11 ounces, weight Fresh Udon Noodles; 2 Tablespoons Peanut Oil For Frying; ½ whole Large Cooking Onion, Sliced; 2 whole Carrots, Peeled And Sliced; 2 stalks Celery, Sliced; 2 teaspoons Sugar; 2 Tablespoons Peanut Butter; ½ whole Red Bell Pepper, Sliced; ½ whole Green Bell Pepper, Sliced; 1 cup Snow Peas, Trimmed; 1 cup Unsalted Cashews; Chopped Cashews And Green Onions, For Garnish (optional)