Lily's been asking for baked mac and cheese a lot lately, including for breakfast in the morning before school when there's not much time. Her favorite part is also the top, where the cheese is crispy. I've started making what's essentially grilled cheese with a pasta base, under the broiler.
I put cold leftover pasta from the fridge on a quarter sheet [1], and put cheese on top:
Put it in the oven, turn on the broiler, set the timer for ~4min.
The broiler can turn things from "not yet" to "burnt" very quickly, so after the timer goes off if it's not done I bump another minute on the timer (in case I forget) but plan to check it after 30s.
Here's how it looks when it's done:
If you don't like any browned pasta with crunchy bits (I do) you can put the oven rack down a notch.
The broiler is fast enough and the layer of pasta and cheese is thin enough that I don't bother with microwaving anything first.
[1] It's a "quarter" sheet pan (9x13") because in a professional
kitchen full size is 18x26". You can't fit that in a home oven,
though, and you'll often see either 2/3 sheets (16x22"), about the
biggest you can fit on a rack, or half sheets (13x18") which are an
easier to manage size and get better airflow. I like the quarter
sheets for cooking smaller amounts because they're small enough to fit
in our dishwasher and I can put two different things on an oven rack.
Ours are uncoated aluminum, 16 gauge with a wire-reinforced rim, $5/each
in bulk.