Thaw the hash brown potatoes in a colander (you can let them sit on your counter for an hour or so or thaw them in the fridge overnight).
Once the potatoes are thawed, transfer them to a 9x13 casserole dish. Preheat your oven to 375F and move the rack to the top third of the oven.
If using sausages (vs. sausage meat), take the meat out of the casings and crumble it into a skillet. Cook, breaking up the meat as it cooks, over medium-high heat until browned (about 8-10 min). Transfer it to the casserole dish and leave the remaining fat in the skillet (but if it's more than 1-2 tablespoons, drain the excess). If the pan is looking dry, add a tablespoon or so of olive oil.
Add the onion, bell peppers, and garlic to the skillet and cook over medium-high heat for 4-5 minutes, then transfer to the casserole dish.
In a large prep bowl, add the eggs, half-and-half, and salt. Whisk until just combined, then stir in the chopped spinach and half the cheese.
Pour the egg mixture into the casserole dish and toss everything together then smooth into an even layer.
Top with the rest of the cheese.
Bake, uncovered, for 45 minutes or until the edges start to brown and the middle is set (insert a toothpick and if it comes out clean, it's done). Don't cook for too much longer than needed as the egg will dry out.
Let it cool for a few minutes prior to cutting and serving. Season individual portions with extra salt & pepper if needed.