Place potatoes in a large saucepan; add water to cover them by 1 inch. Add 1/4 tsp. salt and bring to a boil. Reduce heat to medium-high and cook until tender, about 10 minutes. Drain in a colander.
Warm oil in a large skillet over medium-high heat. Add onion and jalapeño and cook, stirring often, until softened, 3 to 5 minutes. Stir in beans. Add potatoes and cook until warmed through, about 5 minutes. Season with salt and pepper.
Spoon about 1/2 cup filling into each taco shell and serve immediately. Offer rice, salsa, guacamole, sour cream and sliced scallions on the side, if desired.