Serving this Sweet Potato Open-faced allows you to see the array of colorful veggies piled high on the crusty bread. This is a perfect vegetarian lunch or dinner option.
Bring water to a boil in a small saucepan. In a pint sized ball jar or small glass bowl, add vinegar, sugar, salt, and peppercorn. Add boiling water to vinegar mixture and stir until the sugar is dissolved. Submerge the onions in the pickling solution and let sit for at least 30 minutes.
Preheat oven to 450 degrees and line a baking sheet with parchment paper. In a medium bowl, toss the cornstarch, salt, cumin, cayenne, and garlic. Whisk in the olive oil until a paste forms. Toss in sweet potatoes until evenly coated and transfer to the baking sheet in a single layer. Bake for 25 minutes, flipping halfway through. Remove from oven and set aside.
Whisk together the pureed adobe and mayonnaise until completely combined.
Lightly toast the bread and then spread a 1/4 cup beans onto a slice of bread and then top with sweet potatoes, greens, avocado, squirt of lime juice, cilantro, cheese, tomatoes (if using), pickled red onions, and mustard seeds (if using). Drizzle with mayo and season with black pepper. Repeat with remaining pieces of bread. Serve right away.