How to Make It
Preheat oven to 350°F. Melt butter in a Dutch oven over medium. When butter foams, add onions, thyme, salt, and pepper. Cook, stirring occasionally, until onions soften but do not color, about 12 minutes. Remove from heat.
While onions are cooking, bring a small pot of water to a boil over medium-high. Add rice, and cook 5 minutes. Drain and stir rice into onions.
Cover Dutch oven with aluminum foil; cover with lid. Cook in preheated oven 30 minutes. Remove from oven, and let stand, covered, until rice is tender, about 15 minutes. Stir heavy cream and Gruyère into hot onion mixture until cheese is melted and mixture is creamy. Adjust seasoning to taste. Stir in green garlic and parsley until combined.