Step 1

Preheat the oven to 450 degrees. Line a large baking sheet with parchment or a silicone mat.

Step 2

Lightly oil the vegetables (or coat in broth). Arrange the vegetables in a single layer on the baking sheet, with the mushrooms gill-side up. Sprinkle a pinch of salt on everything except the portobellos.

Step 3

Bake for 15 minutes. Remove from the oven, and transfer the two garlic cloves to the bowl of a food processor. Flip the sweet potato, onion and red pepper. Leave the portobellos gill-side up and sprinkle them with salt. Return the pan to the oven and bake until the vegetables show slight browning and the sweet potatoes are tender, an additional 10 to 15 minutes.

Step 4

In a food processor combine the two cloves of garlic that were removed from the oven, walnuts, salt, and smoked paprika. Pulse until the nuts are very finely chopped. With the machine running, quickly drizzle in the water and balsamic, stopping to scrape down the sides as needed. Taste for seasoning, and add black pepper. The consistency should be a thick, slightly chunky, spreadable dip.

Step 5

Toast the bread. Slice the portobello caps. Spread a layer of walnut sauce on each piece of toast and top with the roasted vegetables. Lightly dress the lettuce with a spoonful or two of the walnut sauce and pile on top of each sandwich.