In a food processor, combine the garlic (squeeze the garlic from the skins if using roasted garlic), fish sauce, lemon juice, mustard, olive oil, mayonnaise, and Parmesan. Whisk to combine and season with freshly ground pepper and additional fish sauce, if desired. To dress salad, toss the romaine lettuce and tomatoes in a large bowl and add the dressing a spoonful at a time, tossing to coat. Refrigerate any remaining dressing and use to dress any salad or as a sandwich spread or vegetable dip.