Make dressing: Whisk together mayonnaise, buttermilk, sour cream, parsley, chives, mustard, salt, pepper, and garlic in a bowl. Cover and chill for 1 hour.
Make “bacon” bits: Preheat oven to 375°F. Place mushroom slices, oil, and salt in a bowl; toss to coat. Arrange mushrooms in a single layer on a baking sheet. Bake until well browned and beginning to get crisp, about 20 minutes. Let cool completely on baking sheet, about 20 minutes. Coarsely crumble mushrooms.
Make salad: Place 1/2 head romaine lettuce on each of 4 plates. Drizzle each with 2 tablespoons dressing; sprinkle evenly with “bacon” bits, tomatoes, and cheese. Refrigerate remaining dressing in an airtight container for up to 1 week.