These are inspired by the street hot dogs they serve in Norway, that come wrapped in a flatbread that is similar to lefse. Lefse is a Norwegian flatbread made with riced potatoes. They sell it here in the supermarket, but if you can't find it you can substitute your favorite flour tortillas—or try making make lefse; I have a fun recipe!
Combine the sour cream, cider vinegar, celery seeds, a pinch of salt and some pepper in a medium bowl. Add the fennel, fronds and onion and toss together. Taste for seasoning and adjust if needed.
Whisk together the mustard and lingonberry preserves in a small bowl.
Heat a cast-iron grill pan over high heat.
Split the hot dogs almost in half lengthwise. Oil the grill pan, then grill the hot dogs on the first side until charred in spots, 2 to 3 minutes. Flip and grill on the second side until charred and heated through, another 2 to 3 minutes.
Add each hot dog to a warmed piece of lefse, then top with the lingonberry mustard sauce and fennel slaw.