Poutine on the Ritz Burger

Place mushrooms in a small heatproof bowl and cover with 1 cup boiling water. Let sit until mushrooms are tender, about 20 minutes. Strain through a fine-mesh sieve into another small bowl, reserving soaking liquid and mushrooms separately.

Meanwhile, melt butter in a small skillet over medium-high heat. Cook flour, stirring constantly, until roux is deep golden brown and smells nutty, 2–4 minutes; set aside.

Heat oil in a medium saucepan over medium-high. Cook onion, shallot, leek, and garlic, stirring occasionally, until vegetables are deep golden brown and caramelized, 10–12 minutes. Add tomato paste and cook, stirring often, until slightly darkened in color, 1–2 minutes. Add mushrooms, mushroom liquid, demi-glace, black peppercorns, vinegar, green peppercorns, Worchestershire, parsley, thyme, and bay leaf. Bring to a boil, then reduce heat to low and simmer until slightly reduced and flavors have melded, about 30 minutes. Strain infused stock through sieve into a large bowl, pressing on solids to extract as much liquid as possible; discard solids.

Return stock to saucepan and bring to a boil. Add roux and cook, whisking constantly, until gravy is thick and smooth, 2–3 minutes. Reheat before using.

Do Ahead: Gravy can be made 3 days. Cover and chill.

Generously butter cut sides of buns. Heat a large griddle or cast-iron skillet over medium-high. Toast buns, cut side down, pressing slightly, until deep golden brown, about 2 minutes. Transfer to a plate and loosely wrap with foil to keep warm.

Wipe griddle clean with paper towels, then heat over medium-high until very hot, about 2 minutes; lightly brush with oil. Meanwhile, divide beef into 4 equal portions (do not form patties).

Working in batches if needed, transfer beef to griddle and smash flat with a spatula to form imperfect, loose patties about 4" in diameter (craggy edges are your friend). Season liberally with salt and pepper and cook, undisturbed, until outer edges are brown, about 2 minutes. Turn patties, season with salt and pepper, and cook until medium-rare, about 1 minute.

Transfer burgers to toasted buns. Top with mayonnaise, pickles, a handful of fries, cheese curds, gravy, and chives. Serve remaining fries on a platter topped with additional cheese curds, gravy, and chives alongside.