Preheat oven to 400°F (200°C). Prepare a baking sheet with foil or parchment.

In a small bowl, mix together butter, brown sugar, mustard, garlic, thyme (if using), and a generous pinch of salt and pepper.

Using a sharp, sturdy chef's knife, carefully cut the acorn squash in half, from stem to tip. (I find it easiest to press the tip of the knife in and then pivot down to make a long slit, repeating as needed). Scrape out the seeds and stringy bits inside each squash half using a spoon, until the inside is smooth.

Using a sharp paring knife, score the insides of the acorn squash halves in a cross-hatch pattern, about a half-inch deep cuts (alternately, you could poke some holes all over with a fork). If the halves wobble or don’t sit evenly, shave a small part of the very bottom off with a sharp knife. Place the squash halves cut side up on the prepared baking sheet. Split the butter mixture evenly between the 4 halves and rub it around with the back of a spoon, including the rim.

Place in oven and after 45 minutes, scatter a thyme sprig or two over each half (if using). Bake for another 15-25 minutes until nicely browned around the edges (60-75 minutes, total). The flesh should be very soft and cooked through when you poke it with a fork.

Remove the tray from the oven and let cool for a bit before serving. There will be a delicious buttery sauce in the well which will soak in as the squash sits. If you want to cut it into wedges or slices, cook it at least 30 minutes ahead and slice when the sauce has been absorbed.