50 white mushrooms (about 2 lb/1 kg)
3 tbsp (45 mL) olive oil
1 onion finely diced
1-1/2 cups (375 mL) finely chopped broccoli florets
1 clove garlic, minced
1/2 tsp (2 mL) salt
1/4 tsp (1 mL) pepper
1 cup (250 mL) shredded old Cheddar cheese
1/4 tsp (1 mL) hot sauce
Wipe mushrooms clean with damp cloth; do not rinse. Remove and finely chop stems; set aside.
Place mushroom caps, hollow side down, on parchment paper–lined or greased rimmed baking sheet; brush with half of the oil.
Bake in 400°F (200°C) oven until slightly softened, about 8 minutes. Let cool; pat dry.
In skillet, heat remaining oil over medium-high heat; cook onion, broccoli, garlic, chopped mushroom stems, salt and pepper, stirring often, until softened and no liquid remains, about 5 minutes. Let cool.
In bowl, stir together broccoli mixture, cheese and hot sauce. (Make-ahead: Refrigerate mushrooms and filling in separate airtight containers for up to 2 days.)
Spoon 1 tbsp (15 mL) filling into hollow of each mushroom. Bake on greased or parchment paper–lined baking sheet in 375°F (190°C) oven until golden, about 12 minutes. Serve warm.