1/2 pound sweet Italian sausage
6 ounces fresh baby spinach
2 cups Simple Tomato Sauce (or 2 cups of your favorite gluten-free jarred sauce)
1/2 pound low-moisture fresh mozzarella cheese, sliced by cross-section into 10 rounds
1/4 cup finely grated Parmigiano-Reggiano cheese
Prepare the Whole Grain Focaccia according to the recipe directions through Step 4, but instead of setting the dough to rise before rolling it out, roll it out into a rectangle about 12-inches by 17-inches (and about 1/2-inch thick). Line a 10-inch by 15-inch sheet pan (slightly larger than a quarter sheet) with unbleached parchment paper and transfer the dough to the pan, pressing it into the corners and gathering it up the sides.
Sprinkle the dough with warm water, cover it with plastic wrap, and set it in a warm, draft-free location to rise until it is about 150% of its original size.
While the dough is rising, preheat your oven to 375 degrees F.
Also while the dough is rising, prepare the sausage and spinach. Remove the sausage from its casings and break into pieces. In a large, dry skillet, cook the sausage uncovered over medium-high heat until lightly browned all over and cooked through (about 8 minutes). Once the sausage has nearly finished cooking, turn off the flame and add the fresh spinach leaves to the skillet. Cover the hot skillet tightly and allow the spinach to steam for about 3 minutes. Remove the cover and stir until the spinach is wilted. Cover and allow the spinach to steam further if necessary. Set the skillet aside.
Once the dough has finished rising, top with all of the tomato sauce and spread in a single layer. Scatter the slices of mozzarella cheese evenly over the top of the sauce, and top with the sausage and spinach mixture. Sprinkle the grated Parmigiano-Reggiano cheese evenly over the top of base of the dough.
Place the baking pan in the center of the preheated oven and bake for about 25 minutes or until the edges of the pie are golden brown and the cheese is melted and the sauce bubbling. Allow to cool until the cheese is set (about 5 minutes) before slicing into 8 squares with a pizza wheel or sharp knife. Serve warm.
For a make-ahead meal, bake the pizza for 15 minutes and remove from the oven. Allow to cool completely, then cover and place in the refrigerator. Before serving, bake for another 15 minutes in a 375 degree F oven, and then proceed with serving directions in the previous step.