Mix all ingredients, except liquid and oil, in a large bowl. Use a potato masher, fork, or food processor to mash until chunky, adding a small amount of liquid if necessary. The mixture should be moist enough to hold together, but not wet. Let stand about 10 minutes. Form mixture into four patties and let rest on a plate for a few more minutes; this is easiest to do with wet hands. Bean burgers can be made ahead to this point, cover tightly and refrigerate for up to 24 hours, returning to room temperature before cooking. When ready to cook, heat a large heavy skillet or griddle over medium heat; film with cooking oil. When the oil is hot, carefully add the burgers to the pan and cook without moving until browned, about 5 minutes. Use a large spatula to gently turn the burgers and brown the second side. Serve on buns with favorite burger condiments and trimmings.