Preheat oven to 220 C (425 F). My trick is starting hot so the fries crisp fast while the cheese melts.
Heat 1 tbsp (15 ml) oil in a large skillet over medium-high. Add diced onion and sauté 2-3 minutes until translucent and fragrant.
Add ground beef, sprinkle with salt, pepper, and garlic powder. Break meat into even pieces and cook 6-8 minutes until well-browned and caramelized at edges. You want small browned crumbles that smell rich and toasty.
Drain excess fat, then stir in ketchup and mustard. Simmer 1-2 minutes until sauce is glossy and coats meat. Taste and adjust seasoning; the mixture should taste tangy and slightly sweet.
Spread half the fries in the bottom of a lightly greased 9x13-inch baking dish. Arrange fries so they overlap a bit but form an even base.
Spoon the beef mixture evenly over the fries, distributing onion and pickles. You should see contrasting colors and a saucy sheen over the fries.
Sprinkle half the shredded cheddar across the beef layer. This creates a melty binder and prevents sogginess between layers.
Add remaining fries on top, pressing lightly. Scatter remaining cheese over the fries for a golden, bubbling crust.
Bake at 220 C (425 F) for 12-15 minutes until cheese is melted and edges of fries are crisp and brown. Look for a bubbling cheese rim and toasted potato edges.
Remove and rest 5 minutes. The aroma should be a mix of browned beef, sharp cheddar, and warm potato. Slice and serve with extra pickles or a drizzle of ketchup if desired.