Prep: 1. Preheat oven to 425 (or heat grill). 2. Toss mushrooms with oil and 2/3 cup water; season with salt and pepper. Make: 1. Arrange stem side down on a large rimmed baking sheet; cover loosely with foil. Roast until tender, 30-45 min. (or grill for about 15 min.). 2. In a bowl, mash avocado with a fork. Add mustard, horseradish, 1 tablespoon lime juice, and jalapeño; season with salt and pepper. 3. Toast rolls (if using). To serve, layer each bottom half with tomato and portobello, and each top half with avocado mixture.
Eat with toasted rolls for the classic burger, or wrap in lettuce for a low-carb version.