Slow-Cooker Vegetarian Lasagna

Ingredients:1 large egg1 15- to 16-ounce container part-skim ricotta1 5-ounce package baby spinach, coarsely chopped3 large or 4 small portobello mushroom caps, gills removed (see tip), halved and thinly sliced1 small zucchini, quartered lengthwise and thinly sliced1 28-ounce can crushed tomatoes1 28-ounce can diced tomatoes3 cloves garlic, mincedPinch of crushed red pepper (optional)15 whole-wheat lasagna…