1. Preheat the oven to 350°F.
2. In large mixing bowl, combine all ingredients. Form into 3 equal-sized patties. Chill in the freezer for 15 minutes.
3. Transfer the patties to a baking sheet lined with parchment paper if desired and roast in the oven until the internal temperature reaches 145°F, about 15 minutes.

For the eggplant buns: Preheat the oven to 425°F. Line a baking sheet with foil. Slice one eggplant evenly into 3/4-inch-thick rounds. Arrange in a single layer on the prepared pan. Drizzle 1 1/2 tablespoon of cooking fat evenly over the eggplant, then flip each slice and drizzle 1 1/2 tablespoon of fat on the other side. Season with the salt and pepper. Roast the eggplant for 20 minutes, until it is browned on the outside and fork-tender. Allow to cool, then stack a burger and fillings between two slices, and serve.

For portobello mushroom buns: Rinse 4 mushroom tops with cool water or wipe with a wet paper towel to remove any excess dirt. Dry thoroughly. Place the mushrooms upside-down on the prepared baking sheet. Remove the stems by gently breaking them off, and drizzle 3 tablespoons of melted cooking fat evenly over the mushrooms. Season with the 1/2 teaspoon salt, 1/2 teaspoon pepper, and 1 clove of minced garlic. Roast for 10 minutes, then flip and roast for another 10 minutes, until fork-tender. Allow to cool, then stack a burger and fillings between two mushroom caps and serve.

For sweet potato buns: Peel 2 sweet potatoes and cut into 1/2-inch-thick rounds. (Choose thick, round sweet potatoes over long, skinny ones.) Heat 2 tablespoons cooking fat over medium heat in a large skillet, swirling to coat the bottom of the pan. When the fat is hot, add the largest slices from the middle of the potato, laying them out in a single layer in the pan. (Cook these in batches, if necessary.) Cook until fork-tender and browned, 3 to 5 minutes on each side. Sprinkle with salt and pepper. Let cool, then stack a burger and fillings between two slices of sweet potato, and serve.

Source: Calorie Count [2]