In a large Dutch oven, heat 2 tablespoons butter and olive oil over medium heat. Add green onion; cook, stirring constantly, for 2 to 3 minutes. Add rice; cook, stirring constantly, until very lightly browned, 2 to 3 minutes. Add hot broth, ¼ cup at a time, stirring constantly and allowing liquid to be absorbed after each addition. Repeat procedure until rice becomes creamy in texture, about 30 minutes. Add squashes, carrot, salt, and white pepper; cook, stirring constantly, for 5 minutes. Remove from heat, and stir in spinach, cheese, thyme, and remaining 2 tablespoons butter. Garnish with a thyme spring, if desired.
