One of the dishes I made for my seder got compliments from everybody (and requests for how I did it), and it was incredibly easy. I wasn't expecting it to be one of the stars of the night. So, to share this discovery with others:
Dice a large sweet onion (next time I'll use more) and cube about a pound of butternut squash (~half-inch cubes), mix in enough olive oil to coat, spread in a pan, and roast at 400 for about half an hour (stirring a couple times in there). Meanwhile, cut four red peppers1 in half, removing the stems, seeds, and white vein-like stuff. Ideally you will have selected peppers that are square-ish in shape, such that when you set the halves in a pan they'll stay put rather than tipping over. Fill the peppers with the cooked onion-squash mixture, add a bit of water to the pan (I find this helps prevent the peppers from burning), and put back in the oven until done (maybe another 20 minutes, though definitions of "done" vary). The onions on top should be caramelized and everything should be tender.
That's it. I didn't even season it before cooking, and it turns out I didn't need to.
I cooked this the day before and it went onto a hot plate during the early part of the seder.
At other times of year I might add rice to the mixture, particularly of the multi-colored-mixture variety. There were going to be other starches, so I didn't add farfel.
1 Yellow or orange peppers would work taste-wise, but the colors are prettier with red ones (with the orange squash and the light-yellow onion). Green peppers are never an option in my kitchen, but I also think they'd be too bitter in this combination even for people who like them otherwise.