- Heat 1 tablespoon oil in large nonstick skillet over medium heat.
- Add onions and rosemary and cook stirring occasionally, until lightly browned, about 12 minutes; remove to a bowl.
- Return the pan to the stove and add half the remaining oil.
- When it's hot, add the portabella mushrooms and sauté until nicely browned, about 5 minutes.
- Add portabellas to the onion mixture and repeat sautéing with the remaining oil and the white mushrooms.
- Return everything to the pan and add the tomatoes, tomato paste, garlic, 1-1/2 cups water and vinegar.
- Simmer gently for 12 to 15 minutes, then swirl in the butter.
- Season with salt, pepper and red pepper flakes.