Preheat the oven to 350ºF. Lightly grease a 2 to 3 quart baking dish with olive oil spray and set aside.
Add the olive oil to a large skillet set over medium heat. When hot, add in the onion and cook until tender and starting to brown, about 5 minutes. Stir in the garlic and cook until fragrant, about 30 seconds. Add in the kale and vegetable broth. Cook, stirring occasionally, for about 6 to 8 minutes, until the kale has wilted down and the broth has evaporated. Remove from the heat.
Add the vegetable mixture to a large bowl, along with the quinoa, lemon zest, cannellini beans, Greek yogurt and 1 cup of the cheddar. Mix until well combined. Season with salt and red pepper flakes to taste.
Transfer the mixture to the prepared baking dish. Sprinkle the top with the remaining 1 cup of cheddar cheese and the walnuts.
Bake for about 10 to 13 minutes, until heated through and the cheese is melted.
You can cook the quinoa in advance so it is ready to go come dinnertime. However, be sure to warm it back up before mixing it in with all the other components. Otherwise it will take longer to heat up in the oven and the dish could dry out.