Bake Potato Puffs: Spread the frozen potato puffs on a baking sheet in a single layer. Bake at 425°F for 20 to 25 minutes, or until they are hot and crisp.
Build the Poutine Base: Transfer the crispy potato puffs to a serving platter. Spoon the pulled beef brisket evenly over the top.
Add Mozzarella: Scatter the mozzarella over the brisket and potato puffs.
Melt the Cheese: Place the platter back in the oven for 3 to 5 minutes, just until the mozzarella softens and melts.
Add Gravy and Green Onions: Spoon the warm brown onion gravy over the poutine. Sprinkle the sliced green onions on top.
Prepare Shallots: Chop the shallots finely so they cook quickly and blend into the mushrooms.
Cook Mushrooms: Heat the olive oil in a skillet over medium heat. Add the chopped shallots and sliced mushrooms. Cook for 5 to 7 minutes, stirring often, until the mushrooms soften and release their moisture.
Make Maple Mixture: In a small mixing bowl, whisk together the maple syrup, Dijon-style mustard, and sherry vinegar until smooth.
Glaze Mushrooms: Pour the maple mixture into the skillet with the mushrooms. Stir and cook for about 2 minutes, until the mushrooms are coated and the glaze looks glossy.
Warm the Spinach: Place the baby spinach in a salad bowl. Spoon the warm maple mushrooms over the spinach and toss gently with tongs so the spinach lightly wilts.
Finish Salad: Sprinkle the chopped walnuts over the warm mushroom spinach salad.
Serve: Serve the meat and potato poutine hot with the warm maple mushroom spinach salad on the side.