In a 10" sauté or fry pan, over medium heat, add the diced mushrooms.
Allow mushrooms to release their water, occasionally stirring with a cooking spoon, approximately 4-6 minutes.
Add butter to the mushrooms and stir with a wooden spoon until mushrooms have soaked up the butter and are evenly browned, approximately 3 minutes. The mushrooms will have cooked down to about ½ cup, set aside.
Over high heat, boil 8 cups of water in a large pot.
In a large mixing bowl, add 4-6 cups of cold water and 1-2 cups of ice. Set aside.
Place a colander in the sink.
Once the water is boiling in the pot, add all the spinach and cook until bright green for 1-2 minutes.
Strain the spinach by pouring the contents of the boiling pot into the colander.
From the colander, immediately place the spinach into the ice bath and allow it to cool for 3-5 minutes.
Once cool, strain the spinach back into the colander. Using your hands, squeeze as much of the water out as possible. You could also use a wooden spoon to press the spinach against the sides of the colander to remove any remaining water. You want the spinach to form a solid dry ball.
Finely chop the cooked and strained spinach using a chef's knife on a cutting board or pulse in a food processor 15-20 times. Chopped spinach will amount to approximately ½ cup, set aside.
In a large mixing bowl, combine the chopped spinach, sautéed mushrooms, breadcrumbs, fontina, eggs, chopped basil, salt, pepper, and mix well. Set aside.
Using a flour sifter, fine flour sieve, or a salt shaker full of flour, lightly sprinkle the Mercato ravioli press evenly with flour.
Lay a 10" pasta sheet down on the ravioli mold and gently press the pasta into the indentations.
Using a teaspoon, scoop the filling mixture into the pasta indentations, pressing down with the spoon or finger to ensure it's sufficiently full.
Using a pastry or basting brush, paint the egg wash in between the ravioli squares, making sure you coat all of the edges and in between each ravioli.
Lay the second pasta sheet on top of the filled ravioli and gently press down on the edges to remove any trapped air and seal the edges.
Using the metal rolling pin, roll from end-to-end and side-to-side to perfectly cut the ravioli along the perforated edges.
Lifting the plastic handles, flip the ravioli onto a floured surface.
Flour the ravioli tablet and repeat the process three more times, resulting in 40 ravioli.