Add oil to a large pot and when hot, add the onions. Cook for 5 minutes before adding the mushrooms and garlic. Cook for 10 minutes until browned. Remove the mushrooms from the pot and transfer to a plate.
Add the carrots and leeks to the same pot. Sauté until the veggies begin to soften, about 5 minutes.
Pour in the stock and add the mushrooms back in. Cook until the liquid has reduced by half.
Add the rice, and thyme. Bring the mixture to a boil, lower heat and allow to simmer, uncovered, until the rice is tender, about 45 minutes - 1 hour, stirring occasionally. Add some water to the pot during simmering if needed.
Stir in the creme fraiche and remove the pot from the heat. Season with salt and pepper to taste.