Place potatoes in a large pot of cold, salted water, bring to the boil over medium-high heat, then reduce temperature to low and simmer until potatoes are fork-tender, about 15 minutes. Drain potatoes and let cool 10 minutes.
Meanwhile, in a medium skillet set over medium-low heat, add oil and melt butter, then add onions and thyme and cook until onions are golden brown, about 15 minutes, stirring occasionally. Transfer mixture to a plate and cool slightly.
In a large bowl, mix together dill, mayonnaise, jalapeno and pickled jalapeno liquid, then gently fold in reserved potatoes and onion mixture, season to taste with black pepper and serve immediately.
