Before starting, preheat your broiler to high. In a large pot, add 10 cups warm water and 2 tsp salt (use same for 4 ppl). Cover and bring to a boil over high heat. While water comes to a boil, roughly chop spinach.

Pat chicken dry with paper towels, then season with half the Montreal Steak Spice (all for 4 ppl). Heat a large oven-proof pan over medium-high heat. When hot, add 1 tbsp oil (dbl for 4 ppl), then chicken. Pan-fry until golden-brown, 1-2 min per side. Transfer chicken to a foil-lined baking sheet. Broil in the middle of the oven until chicken is cooked through, 5-6 min. Carefully wipe the pan clean.

While chicken bakes, add fusilli to the pot of boiling water. Cook uncovered, stirring occasionally, until tender, 10-12 min. Reserve 1/2 cup pasta water (dbl for 4 ppl), then drain and return fusilli to the same pot, off heat.

Re-heat the large pan (from step 2) over medium. When hot, add 2 tbsp butter (dbl for 4 ppl), then onions, garlic puree and spinach. Cook, stirring often, until spinach is wilted, 2-3 min. Add cream cheese and cream. Cook, stirring often, until cream cheese melts, 1-2 min.

Add fusilli and reserved pasta water to pan with the spinach sauce. Season with salt and pepper, then stir to combine. Remove pan from heat. (NOTE: If you do not have an oven-proof pan, transfer mixture to a lightly oiled 8x8-inch baking dish. Use a 9x13-inch baking dish for 4 ppl.) Sprinkle with mozzarella. Broil in the top of the oven until cheese melts, 4-5 min.

Slice chicken. Divide chicken and pasta between plates. Sprinkle chili flakes over pasta, if desired.