Preheat oven to 350°F.: The air in the oven warms and creates steady heat, producing an even bake that sets the custard and browns the top. You should feel a warm, consistent flow when you open the door and see the racks gently glowing, not scorching. Ensuring the oven reaches temperature prevents undercooked centers, a frequent mistake, so give it time to stabilize before placing the dish inside.
Bring the water to a boil in a large saucepan and add the grits and salt.: As the water hits a rolling boil you will see lively bubbles and steam rising, and adding the grits then begins the thickening process immediately. Stir briskly to prevent lumping, and listen for the subtle change from rapid boil to a calmer simmer; that tells you to reduce heat. Adding grits to water that is not fully boiling or failing to stir often leads to grainy clumps.
Reduce the heat to a simmer, cover and cook grits for 5 minutes or until thickened.: At a simmer the surface will show gentle ripple and the aroma of cooked corn will emerge. Covering helps the grits absorb liquid evenly; steam forms tiny beads on the lid which is a good sign. Stir once or twice to check texture. The proper thickness is spoonable yet not dry; overcooking or leaving at high heat can make them gluey, so keep a patient eye on the pot.
Remove grits from the heat and stir in the butter, garlic cheese and 1 cup of cheddar until melted.: Once off the heat, the butter and cheeses melt luxuriously into the warm grits , releasing a savory perfume from the garlic cheese and a sharper cheddar note. You should see glossy swirls and hear a soft, settling hiss as the mixture cools slightly. Stirring gently ensures a homogenous texture. A common misstep is rushing this step while the pot is too cool, which prevents full melting and creates uneven pockets of cheese.
In a separate bowl mix the egg and milk then add to the grits mixture, stirring briskly until combined.: The combined eggs and milk should be smooth and slightly frothy when whisked, and when incorporated the mixture will darken minutely and become silkier. Start by tempering if the grits are piping hot, adding a spoonful of the grits into the egg mixture to warm it, then fold back in, which avoids curdled eggs. Listen for a gentle thickening sound and watch the mixture gain body. Adding cold eggs directly to very hot grits can scramble them, so tempering prevents that.
Pour into a buttered 11 x 7 inch casserole dish and top with remaining 1 cup cheddar cheese.: As you transfer the custardy grits , notice the glossy, spoonable texture settling into the dish. Level it gently with a spatula and sprinkle the remaining cheddar cheese evenly for uniform browning. The visual should be a smooth surface dotted with cheese that will melt and bubble into a golden crust. An error to avoid is overcrowding the dish which can change bake time and texture.
Bake for 1 hour or until golden and bubbly and serve.: In the oven you will hear soft bubbling and see steam waft through the golden, blistered cheese top. The aroma becomes rich and inviting, with toasted cheese notes. The center should be set but still slightly custardy when you jiggle the pan. Let it rest briefly before cutting so the structure firms. The usual mistake is removing it too soon, which leads to a loose center that does not hold slices well.