To make ahead: Bake the bites up to 3 days in advance. Let them cool completely, then cover and refrigerate in the baking dish or transfer to an airtight container.

To reheat: Place in a preheated 300 degree f oven for 10 to 15 minutes, or in the microwave for 15 to 20 seconds, just until warmed through.

To store leftovers: Place in an airtight container in the refrigerator for up to 3 days. Bring to room temperature or reheat per the instructions above.

Tips:
1. Shred your own cheese. Skip the pre-shredded cheeses. It often contains anti-caking agents that affect how well it melts, and it’s typically drier, which can throw off the texture. Freshly grated sharp Cheddar gives the best flavor and consistency. And, for this amount of cheese, I recommend using a food processor with a shredding disc—it’s faster and easier.
2. Grate cheese cold. It’s much easier to handle when it’s straight from the fridge, and it grates more evenly.
3. Drain everything well. Green chiles, jalapeños, and pimientos should be fully drained. Extra moisture can keep the bites from setting up properly.
4. Squeeze moisture from the onion. Even a small amount of liquid from the grated onion can affect the texture, so give it a quick press in a paper towel or fine mesh strainer.
5. Mix thoroughly. For even flavor and appearance, make sure everything is well combined before adding to the baking dish.
6. Smooth the top. Press the mixture gently into an even layer so the bites bake uniformly and cut cleanly.