Stuff the onions into a pint-size mason jar or other heat-safe glass container.
Combine the remaining ingredients in a small saucepan, and heat them until they boil, stirring until the sugar dissolves. This should only take a few minutes. Turn off the stove, and pour the hot brine over the onions. The heat from the brine will soften the onions.
Set aside to cool while you prepare the rest of the tacos.
Make the beans
In a small saucepan, heat the oil on medium high heat. Add the onion and garlic and a pinch of salt and cook until the onion starts to brown. You can add a small splash of water, if needed, to prevent sticking.
Add the paprika (or chili powder and liquid smoke), broth, and beans to the pan, and simmer for 10 minutes, partially covered. Add more water or broth, if needed, if the pan starts to go dry. When the 10 minutes are up, taste and season with salt and pepper, if needed.
To make one taco
Place a tortilla onto the plate and use a slotted spoon to scoop some of the pinto beans onto the tortilla. Top with some pickled onions and the remaining toppings and serve with a lime wedge on the side.
Video
Notes
To warm the tortillas, stack 4 tortillas, wrap them in a damp paper towel, and microwave for 1 minute. Repeat with the remaining 4 tortillas. You can wrap them in foil in two stacks of 4 and warm them in the oven at 350° F for 10 minutes instead. Store leftover taco elements separately. The pickled onions keep for 1-2 weeks in the fridge. The beans and toppings will keep separately for up to 3 days in the fridge. Or, you can freeze the beans for up to 3 months.