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.