For the honey mustard sauce: In a bowl, combine the mayo, mustard and honey; mix well.
For the "sweet pea" burgers: Preheat the oven to 400 degrees F.; line a baking sheet with aluminum foil.
Set the sweet potatoes on the baking sheet and bake until soft, 1 hour. Remove from the oven and let cool.
When cool enough to handle, halve the sweet potatoes, then scoop the flesh into a medium bowl and mash with the garbanzo beans. Add the egg, chili powder, flour, breadcrumbs, Parmesan and some salt and pepper. Mix until fully combined. Form the mixture in 6 patties.
In a large skillet over medium heat, heat the oil and butter, swirling them together. Add the patties to the skillet and cook until brown and crispy, about 5 minutes per side. Transfer to a paper-towel-lined plate to drain.
Serve the burgers on toasted onion rolls spread with honey mustard sauce. Top with lettuce and avocado.