Put potatoes into a pot; cover with salted water by 2". Boil; reduce heat; simmer until tender, 14–15 minutes. Drain; cool slightly. Peel potatoes and break in half; transfer to a bowl. Add cider vinegar, pumpkin seed oil, 1⁄4 cup warm water, shallots, and salt and pepper. Toss gently; cover and set aside.
Pour canola oil into a pot to a depth of 2"; heat over medium-high heat to 350°. Put 1 cup flour into a bowl. Toss a handful of onions in flour and shake off excess; fry until golden, 3–4 minutes. Using a slotted spoon, transfer to a paper towel–lined plate; season with salt. Repeat with remaining onions.
Working with 1 piece of beef at a time, pound it between 2 pieces of plastic wrap with a meat mallet until 1⁄4" thick. Put 1 cup flour in a shallow dish. Season beef with salt and pepper; coat only one side of each piece with 1⁄2 tbsp. mustard. Dredge mustard side in flour; shake off excess; transfer to plates.
Heat 3 tbsp. canola oil in a skillet over medium-high heat. Add half the beef; cook, turning once, until browned, 4–5 minutes. Transfer to a plate; keep warm. Clean out skillet; repeat with another 3 tbsp. canola oil and remaining beef. Return skillet to medium heat; whisk in remaining flour; cook for 30 seconds. Whisk in broth; cook until thickened, about 2 minutes. Remove from heat; whisk in butter 1 piece at a time. To make smooth, strain sauce into a bowl. Stir in balsamic vinegar; season with salt and pepper. Serve steaks with sauce and onions alongside the potato salad garnished with parsley.