Position racks in the upper and lower thirds of the oven; preheat to 450 degrees F. Spread the garlic and all but 2 red onion wedges on a rimmed baking sheet (reserve the remaining onion wedges for the salsa). Spread the sweet potatoes and celery root on a separate rimmed baking sheet. Drizzle each pan with 2 tablespoons vegetable oil and season each with 1/4 teaspoon salt and a few grinds of pepper. Roast, flipping the vegetables and switching the pans halfway through, until the vegetables are caramelized, 25 to 30 minutes. Let cool slightly.
Meanwhile, finely chop the reserved red onion wedges, the pineapple and serrano (remove the seeds for less heat); transfer to a medium bowl. Add half of the cilantro and the lime juice and season with salt and pepper.
Squeeze the roasted garlic from the skins into a separate medium bowl; mash. Stir in the beans. Cover with plastic wrap and microwave until warmed through, 2 minutes.
Warm the tortillas as the label directs. Spread each tortilla with some of the beans and top with the roasted vegetables, pineapple salsa and the remaining cilantro.