Place lemon juice, garlic, mustard, mayonnaise, anchovies, red wine vinegar, salt, and a good amount of freshly ground black pepper in a blender and blend until smooth. With the motor running, slowly add the oil and blend until emulsified. Add the grated cheese and blend for a few seconds to incorporate.
Place spinach in a large bowl, add dressing, and toss to coat each leaf. Add Flatbread Croutons and toss again. Top with shaved Parmesan and serve immediately.
Flatten flatbread dough, gently press into a large rectangle, brush liberally with olive oil, season with salt and pepper, and throw on grill. Grill on both sides until golden brown. Cut into strips about 2 to 3 inches long and 1/4-inch wide.;
Mix water and yeast in a large bowl and let stand 5 minutes to proof. Gradually pour in 2 cups of the flour and stir to incorporate. Mix for about 1 minute to form a sponge. Let stand, covered, for at least 1 hour.
Put sponge in the bowl of a stand mixer. Using the dough hook, add the salt and oil, then add the flour, 1/2 cup at a time, to form a dough. Remove from bowl and knead. Place in a clean oiled bowl and let rise, slowly, about 2 1/2 hours. Divide dough into 4 balls, if desired, let rise again for 1/2 hour, and then roll out as desired.