Place the pancetta in a large, deep skillet, and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on a paper towel-lined plate. Chop into small pieces.
Heat the olive oil in a skillet over medium-high heat. Cook and stir the shiitake and crimini mushrooms in the hot oil for 5 minutes. Reduce heat to medium and add the garlic, olives, capers, lemon juice, and balsamic vinegar; cook another 8 minutes.
Combine the spinach, arugula, bacon pieces, and the mushroom mixture in a large bowl; toss until the spinach and arugula leaves are coated. Sprinkle the feta cheese over the salad; season with black pepper to serve.