Melt butter in a large Dutch oven over medium-low heat. Add onion; cook 8 minutes. Add garlic; cook 4 minutes, stirring occasionally. Increase heat to medium-high. Add rices and thyme; cook 1 minute, stirring frequently. Stir in stock, salt, pepper, and bay leaves; bring to a boil. Reduce heat to low, cover, and simmer 1 hour or until rice is tender (do not stir). Remove pan from heat; let stand 10 minutes. Remove bay leaves; discard. Fluff rice mixture with a fork. Gently stir in parsley.