Clean, trim and slice mushrooms. Sauté in butter until soft and liquid has evaporated. Let cool slightly.
Add mushrooms to a bowl, along with mayonnaise, lemon zest and thyme. Stir to combine.
Slice baguette into 16, ½-inch slices. Layer bread onto a sheet pan. Spoon heaping tablespoons of mushroom mixture onto each slice of bread.
Place canapes in oven on top shelf and broil for about three minutes or until brown and bubbly. Keep an eye on the canapes to ensure that they don't burn!