In large stock pot or Dutch oven, warm oil over high heat. Add mixed mushrooms (except chopped porcini mushrooms) and allow to sear, stirring only very occasionally, until well browned, about 6 minutes. Turn heat down to medium and stir in onion. Cook, stirring often, for 3 minutes before adding reserved chopped porcini mushrooms, minced garlic, and tomato paste. Continue to cook, stirring often, for 2 minutes. All vegetables should be well covered in tomato paste and some should be sticking to the bottom of the pot.