Peel and cut potatoes into large, even chunks. Place in a pot, cover with cold water, add a pinch of salt, and bring to a boil. Simmer until tender, about 15–20 minutes. Drain well and return to the pot.

While potatoes cook, melt 2 tablespoons butter in a skillet over medium heat. Add diced onion and sauté until translucent and soft, about 4–6 minutes.

Mash the drained potatoes in the pot until mostly smooth. Stir in the sautéed onions and sour cream. Season generously with salt and black pepper. Keep the filling warm.

Blanch the Roma tomatoes in boiling water until skins split, then transfer to ice water. Peel the skins, core and rough-chop the tomatoes, and blend with 1/4 teaspoon salt and the jalapeño until slightly chunky. Refrigerate until serving.

Heat corn tortillas in a dry skillet or a damp paper towel in the microwave for 20–30 seconds to make pliable.

Spoon 2–3 tablespoons of potato mixture onto each tortilla, fold or pinch edges to form tacos. Don’t overfill.

Heat oil in a skillet to about 350°F. Carefully add each filled taco and fry 1–2 minutes per side until crisp and golden. Transfer to paper towels to drain.

Serve immediately with fresh salsa and desired toppings.