Bring large pot of water to boil. Meanwhile, core stem ends of tomatoes, then light cut "X" on opposite ends. Cook tomatoes in boiling water until skins begin to pull away from flesh, about 1 minutes. Drain and run under cold water; peel and discard skins. Return pot to stove, add EVOO and onion and cook until softened, 5 minutes. Add tomatoes and crush with masher. Cover and cook over low heat for 20 minutes. Remove from heat and add mint; season.