1. Clean the mussels under cold, running water. Remove any hair connected to the mussels and discard any mussels that are completely or even slightly open. Set aside.
2. In a large skillet or medium-size saucepan over medium-high heat, place 1 to 2 cups (250 to 500 mL) of cold water and bring it to a boil. Place the mussels in the saucepan, cover, and steam for 2 to 3 minutes, or until most of them have opened. Transfer the mussels and their juice into a large bowl. Discard any mussels that did not open. Once the mussels have cooled, grip the top shell in one hand  and the bottom shell in the other hand, and give a small pull to detach the shells. Using your fingers or a small fork, remove the meat from the shell and place it in a small bowl. Repeat with the remaining mussels and set aside.
3. In a separate skillet, melt half of the butter over medium heat. Add the mushrooms and fresh thyme, and cook for 1 to 2 minutes, until the mushrooms turn slightly brown. Add the mussel meats, stirring occasionally, and cook for 2 to 3 minutes, or until the liquid has reduced by at least half.
4. In the meantime, toast the bread. Spread each slice  with the remaining butter and 1 tsp (5 mL) of the mustard. Spoon a generous portion of the mussel meat mixture onto each slice of toast. Top with pepper, some grated cheese and the remaining thyme leaves.
5. Serve immediately so that the toast doesn’t get soggy.