Pre-heat your oven to 375 degrees.
Start by cooking the quinoa. Place the quinoa and chicken stock in a small pot and bring to a boil. Lower to a simmer, cover with a lid, and cook for 15 minutes until all the liquid has been absorbed. When cooked, let the quinoa rest, covered, for 10 minutes and then fluff it with a fork.
While the quinoa is cooking, prepare your vegetables. Place the broccoli florets and chopped red onions on a foil-lined baking sheet. Drizzle with olive oil, and add a pinch of salt and the red pepper flakes. Toss to coat. Roast your vegetables for 15 minutes until just tender.
Transfer the cooked quinoa to a large bowl. Add the shredded chicken, diced tomatoes and 1 cup of the cheddar cheese. Use a spatula to combine the ingredients. The cheese should get nice and melt-y. Fold in the roasted broccoli and onions. Taste the mixture and season with salt and fresh ground pepper to taste.
Transfer the quinoa mixture into a 9x9 inch baking pan (or whatever size pan you happen to have, obviously) and smooth the top. Sprinkle the remaining ¾ cup cheddar over the quinoa and add the panko in an even layer, if you’re into the whole “crispy topping” thing.
Bake for 25 minutes until the cheese is melted and the breadcrumbs are lightly browned. Serve warm.