Set the salmon filets in a small, shallow, greased dish- skin side down. In a small bowl, whisk together the oil, lemon, oregano, mustard, and garlic. Drizzle the marinade over the salmon. Sprinkle with salt and pepper. Set the salmon aside to marinate (room temp is fine- as long as it isn't super warm in your house).
ROAST THE TOMATOES AND SALMON:
Lay the tomato slices onto a greased, rimmed pan. Brush with olive oil and sprinkle with salt and pepper. Place the tomatoes in the oven and set the timer for 10 minutes. Check on the tomatoes and make sure they are not roasting too quickly- they'll start to shrivel up and will appear dark on the edges- the juices may burn around the sides. If they appear to be just as roasted as they can get before burning, go ahead and take them out and let them cool. Otherwise, keep them in the oven.
Add the dish of salmon to the oven, too. Continue to keep an eye on the tomatoes until they are sufficiently roasted. The salmon will bake anywhere from 8 to 15 minutes, depending on the thickness of your salmon. It'll be done when the fish flakes away easily with a fork.
Serve each salmon filet accompanied by several roasted tomatoes and oregano sprigs for garnish, if desired.
Notes
The roasting of the tomatoes will vary. Thickness can change the roasting time for sure. They can go from perfect to burnt quickly, so just keep a close eye.
If you are counting WW Points, I suggest preparing the tomatoes with a light spray of nonstick spray instead of 1 tablespoon oil. You'll reduce your points that way!