Start by removing the salmon fillet skin and set aside.
In a small bowl, add the butter, parsley to taste and mix everything very well until combined.
Spread all this butter over the salmon fillet, transfer it to a piece of plastic wrap and wrap it up.
Close the ends tightly, put it in the fridge and let it chill for 1 hour.
After this time has passed, cut the salmon into 4 equal pieces and set aside.
In a large skillet over medium to low heat, add the butter and stir until melted.
Add the salmon rolls we prepared earlier and fry them until golden brown on both sides.
Remove them from the heat and set aside.
In the same skillet, add the onion cut into strips, the crushed garlic cloves and sauté until slightly golden brown.
Add one red pepper cut into strips, the mushrooms cut into smaller pieces and sauté for another 2 minutes.
Then, add the spinach leaves and stir until combined.
Add the water and the heavy cream.
Season with the salt, a pinch of black pepper and stir once more.
Add the salmon rolls and the slices of lemon.
Cover the skillet and let everything cook for another 10 minutes.
Serve.