Summer Vegetables in Spiced Yogurt Sauce

3 tablespoons neutral oil or ghee 1 (2-inch) piece ginger, peeled and grated 4 garlic cloves, minced ½ teaspoon cumin seeds ½ teaspoon ground turmeric ⅛ teaspoon ground cayenne 1 serrano chile, thinly sliced (use less, or remove seeds, for a milder stew) 1 medium onion, diced small Kosher salt ½ teaspoon garam masala 4 cups diced summer squash, such as zucchini, pattypan or romanesco, cut into 3/4-inch pieces 3 cups baby turnips, halved or quartered 2 cups plain whole-milk yogurt ½ cup almond flour 2 cups shelled peas or cut green beans 1 ½ cups fresh corn kernels (from 3 ears corn) 5 ounces baby spinach Roughly chopped cilantro leaves, for garnish Cooked basmati rice, for serving (optional)