Mashed Potato and Wild Rice Stuffed Mushrooms

Name: Mashed Potato and Wild Rice Stuffed Mushrooms
Ingredients: 1/2 c. Whole Wild Rice; 12 Large Baby Portabella Caps (Mine Were 2 1/2 To 3 Inches In Diameter Before Cooking); 1 1/4 c. Prepared Mashed Potatoes, Preferably Firm (Packaged Works Well Too); 1 tbsp. Unsalted Butter; 2 cloves (Large) Garlic, Minced; 1/4 c. Finely Chopped Yellow Onion; 2 tsp. Finely Chopped Fresh Thyme, Plus More For Garnish; 1 tsp. Finely Chopped Fresh Rosemary; 1/2 tsp. Kosher Salt; 1/4 tsp. Black Pepper; 1 c. Shredded White Cheddar Cheese, Plus More For Topping; 1/4 c. Grated Parmesan Cheese, Plus More For Topping