1 tablespoon (15 millilitres) pickled jalapeño liquid, or to taste
4 teaspoon (20 millilitres) salt
Cracked black pepper
Place potatoes in a large pot of cold salted water, bring to a boil over medium-high heat, then reduce heat to low and simmer until potatoes are fork-tender, about 15 minutes. Drain and let cool for 10 minutes.
Meanwhile, in a medium skillet over medium-low heat, heat oil and melt butter. Add onions and thyme and cook, stirring occasionally, until onions are golden brown, about 15 minutes.
Transfer onions to a plate and let cool slightly.
In a large bowl, stir together dill, mayonnaise, pickles, pickled jalapeños, pickled jalapeño.