1. Place dried mushrooms and stock in a 2-quart pot, bring to a boil, and then lower heat to a very low simmer. (This makes a mushroom broth.)
2. In a separate 2-quart saucepan over medium heat, combine oil and 1 tablespoon butter. When hot, add onion, leeks, fennel and garlic. Sauté 5 minutes, then season with salt and pepper to taste.
3. Add the fresh mushrooms to pan in 3 batches (wait to add new batch until previous batch has “cooked down,” about 5 minutes).
4. Add rice, mix well to coat, and cook 3 additional minutes; the mixture might be a bit dry. Keep stirring so it doesn’t brown or stick to pan. Then, add thyme and mix again.
5. Add 1/2 cup of the mushroom broth—with bits of dried mushroom—to the pan and cook, stirring until it is almost dry. Then add another 1/2 cup of broth. Repeat until rice is tender but still al dente, about 18 to 20 minutes, checking seasoning as it cooks. (The key to creamy risotto is to continue stirring until it is just about dry before adding more broth. You will end up using most of the broth.)
6. Remove pan from heat and add last tablespoon of butter and grated Parmigiano-Reggiano cheese. Mix well, cover 3 minutes, check seasoning again and serve.