Pour lukewarm water (80-90°F) and sugar into a medium bowl. Sprinkle yeast over water and let stand 4 minutes or until dissolved and foamy.
Add 1 cup of the white flour, 1 cup of the whole wheat flour, olive oil, and salt to the water mixture. Knead by hand or with a dough hook for 5-8 minutes or until dough becomes soft and pliable. If too sticky, add remaining flour by the tablespoon, alternating between white and wheat. Let rise in a warm place for 30-45 minutes or until doubled in size.
Preheat oven to 425°F.
Roll the dough out onto a lightly floured surface to approximately 1/4 inch thick. Cut with a large heart-shaped cookie cutter. Layer half of the hearts with a thin layer of marinara sauce, grated mozzarella, and roughly chopped spinach, leaving a small margin of dough around the edges. Top with the remaining hearts and crimp the edges with a fork.
Place on a baking sheet and bake for 10-15 minutes or until cooked through and lightly browned.
Meanwhile, add spinach, sliced strawberries, and feta to a large bowl; toss to combine.
Serve calzones with additional marinara sauce for dipping; serve salad on the side.