Fill a large bowl with very hot tap water. Add the cornhusks; submerge until softened (weigh down with a dinner plate), about 20 minutes. Transfer to a towel to drain.
Cook half the spinach with 1/4 cup water in a large skillet over medium heat until wilted, about 5 minutes. Add the rest of the spinach to wilt. Drain in a colander; let cool. Squeeze out any excess liquid.
Chop the spinach with a bench scraper. Chop the onion with a knife; add to the skillet with 2 tbsp. EVOO over medium heat. Cook, stirring, until golden, 6 minutes. Add the spinach; season.
Mix the masa, 1 1/4 tsp. salt and 1/2 tsp. pepper in a large bowl. Stir in the broth. Beat in 1 1/2 cups EVOO with a wooden spoon until smooth and creamy.
Plop about 1/4 cup masa at the wide end of a cornhusk. Spread into a 3-by-4-inch rectangle, leaving a 1-inch border at the sides and bottom.
Cut the mozzarella into 1-by-3-inch sticks. Top the masa rectangle with a few shreds fo chicken, a stick of cheese and 1 tbsp. spinach.
Fold in the sides of the husk, enclosing the filling, then fold up the end, making a 4-inch package. Repeat with the remaining husks, masa and fillings.
Place a steamer basket over a pot with 2 inches of briskly simmering water. Nestle in the tamales, open ends up. Cover and cook for 1 hour.
Remove the tamales and let cool for a minute. Carefully open them up. Serve warm, with the salsa and sour cream.