Prepare risotto while mushrooms are roasting. Heat oil in a dutch oven or other oven-safe pot with a lid over medium-high heat. Add in onion and cook for 4-6 minutes, or until onion softens.
Add in rice and stir constantly for about 2 minutes until some of the rice begins to turn translucent.
Pour in the white wine or vermouth and cook while stirring until most of the liquid is absorbed - about 2 minutes. Add in 1/2 tsp salt and pepper.
Carefully pour in 2 1/2 cups of the vegetable broth and set remaining 1/2 cup aside (we will use this later). Bring pot to a simmer, place the lid on the pot and put into the oven on the middle rack for 18 minutes until most of the broth has been absorbed.
Carefully take pot out of the oven, remove the lid and heat risotto over medium heat. Add in the remaining 1/2 cup broth and cook risotto while stirring constantly until rice is almost done and sauce has a creamy consistency, 3-5 minutes.
Turn off the heat and stir in parmesan, lemon juice, zest and butter. Stir to combine and taste for salt. Add in more if needed.
To plate, add risotto to bowls, top with tomatoes and mushrooms and garnish with dill. Serve with more lemon wedges if desired.