Place the tomatoes on a baking dish and drizzle with a little extra-virgin olive oil and season with some salt and pepper and transfer to the oven. Roast until the tomatoes start to burst, about 10 minutes. Remove from the oven and let cool. Once cool, transfer to a salad bowl and slightly smash up the tomatoes with a fork, add the minced red onion and the mustard and vinegar. In a slow steady steam, pour in the extra-virgin olive oil, to taste, while mixing with a fork. Add the romaine lettuce and toss to combine.