Grains and Greens Casserole

olive oil plus more for pan

medium onion, finely chopped

heads baby bok choy, sliced into 1/2-inch pieces

bunch kale, stems removed, coarsely chopped

bag (10-ounce) baby spinach

garlic, minced

cooked brown rice (or barley, lentils or quinoa or a combination)

salt and freshly ground black pepper,

large eggs, beaten

shredded Swiss cheese