Combine sliced red onion, vinegar, warm water, sugar and pickle salt in a bowl. Let stand for 20 minutes, stirring twice, then drain thoroughly; pickling is included in prep time.
Mix beef gently with cumin, ancho chile, garlic powder and beef salt. Shape into 12 thin patties a little wider than the slider rolls.
Spread butter over the cut roll faces. Toast them in a large skillet over medium heat in 3 batches for 1 to 2 minutes per batch, then transfer to a platter.
Brush the same skillet with oil and cook patties in 3 batches of 4 for 3 minutes per side until each reaches 160°F. Transfer cooked patties to a clean plate.
Warm refried beans with their measured water in a small saucepan over low heat for 3 minutes, stirring until spreadable. Divide the bean spread among the 12 roll bottoms.
Top each bottom with one cooked patty, a share of queso fresco and a few drained pickled-onion slivers. Close with the roll tops and serve 2 sliders per person.