Remove stems from mushrooms and toss in olive oil. Set aside.
Preheat oven to 350F degrees. Heat oil in an oven safe pan and saute onions for 2 minutes. Add the garlic, tomato paste and saffron and cook for 2 more minutes. Add the rice and cook for a few more minutes.
Add the vegetable stock along with the peas, tomatoes and mushrooms. Stir, cover and cook in the oven for 15 minutes. Take a peek and add more broth if the rice looks dry and stir if necessary. Keep uncovered and cook another 5 minutes. Season with salt and black pepper.