For the grits:
2 1/2 cups water, plus more as needed
2 cups milk
1 teaspoon fine salt
1 cup grits
1/2 cup shredded sharp cheddar cheese
For the topping:
1 tablespoon olive oil
1/2 medium onion, finely chopped
8 ounces uncooked chicken, turkey, or pork Italian sausage, casings removed
1 (5- to 6-ounce) bag baby spinach
Shredded cheese, thinly sliced green onion, or minced fresh chives, for topping (optional)
For the grits: Bring the 2 1/2 cups water, milk, and salt to a boil in a medium saucepan over medium heat. Slowly pour in the grits while constantly whisking. Reduce the heat to medium-low and whisk frequently until the grits and liquid are completely incorporated together, about 5 minutes.
Switch from the whisk to a wooden spoon and cook, stirring every minute or two (make sure to scrape down the sides and get into the corners of the pan), until the grits are thick and creamy, about 45 minutes. The grits will go from a grainy texture to a very smooth texture when you taste it. If the grits start to look dry and clumpy, stir in more water a tablespoon at a time until it smoothes out. Meanwhile, make the topping.
For the topping: Heat the oil in a large frying pan over medium heat. Add the onion, season with salt, and cook, stirring occasionally, until softened, about 5 minutes. Add the sausage and cook, breaking the meat up into small pieces with a wooden spoon, until cooked through and no longer pink, about 5 minutes. Add the spinach and cook until wilted and the liquid has evaporated, about 4 minutes. Remove from the heat and set aside.
To assemble: When the grits are ready, stir in the cheese until completely melted and incorporated. Taste and season with salt as needed.
Spoon half of the sausage-spinach mixture into 4 (1-pint) mason jars. Spoon half of the grits over the mixture (about 1/2 cup per jar). Repeat with layering the remaining sausage-spinach mixture and grits. Sprinkle with the toppings of your choice. Let cool completely, then seal and refrigerate for up to 3 days.
To reheat, remove the lid and cover the jar with a damp paper towel. Microwave until heated through, gently stirring it every 30 seconds.