Heat 1 tbsp oil in a large skillet over medium heat. Add 1 red onion, chopped, and cook stirring occasionally, until soft approx. 6 to 8 minutes. Add 2 small yellow squash, cut into 1⁄2-inch pieces, 2 cloves garlic, chopped, 1 jalapeño, seeded and thinly sliced, 1/2 tsp salt, and 1/4 tsp pepper. Cook, stirring occasionally, until the squash begins to soften, 3 to 4 minutes. Stir in (1) 28-ounce can diced tomatoes and their liquid. Season (4) 6-ounce pieces skinless halibut with salt and pepper and nestle it in the vegetables. Cover and simmer over medium-low heat until the halibut is opaque throughout and beginning to flake, 10 to 12 minutes.