Preheat oven to 425° F and place tomatoes and onion in a 15x10x1 inch baking pan lined with foil. Drizzle with oil, sprinkle with salt and pepper. Roast for 10-15 minutes, stirring occasionally.
Sprinkle the salmon fillets with salt and pepper. Grease a large cast iron skillet and heat over medium-high heat. Place fillets in pan and cook for 3 minutes on each side. Remove from pan.
Add garlic to the pan and stir for 1 minute. Add broth, and stir occasionally. Bring to a boil and cook until liquid is reduced to half, about 1-2 minutes. Add in roasted tomatoes, onion and the salmon to the pan. Cook for 5-7 minutes.