Preheat oven to 350°F.
Toss onion, soy sauce, garlic powder, and 1 Tbsp. oil on a large rimmed baking sheet or cast iron pan to coat. Season with salt.
Bake, tossing once halfway through, until onion is browned around the edges and very tender and beginning to caramelize, 45 minutes.
We usually do this the day before, refrigerate overnight, then warm briefly in the microwave when ready to assemble the sandwiches.
Meanwhile, toss basil, sriracha, and remaining 1 Tbsp. oil in a small bowl. Season with salt, if needed.
Melt butter in a small nonstick skillet over medium heat.
