In 4-quart Dutch oven, melt butter over medium-high heat. Add onions and garlic; cook 5 minutes, stirring occasionally. Stir in wild rice, thyme and 5 cups of the broth. Heat to boiling over high heat, stirring occasionally. Reduce heat to low. Cover; simmer 10 minutes. Stir in brown rice and celery. Cover; simmer 40 to 50 minutes longer or until rice is tender and liquid is absorbed, adding more broth by 1/4 cupfuls if necessary to prevent scorching.