Saute baby spinach in 1 tablespoon sunflower oil, salt to taste and set aside.
In a large bowl combine onion, chicken, egg, parsley, breadcrumbs, water, mustard, Worcestershire sauce, garlic powder, pepper and salt. Use your hands to really incorporate all the ingredients into the chicken evenly.
Lay out a large sheet of plastic wrap on the counter and press the chicken mixture into a rectangular shape. Lay the baby spinach down in the middle of the rectangle, horizontally. Now use the plastic wrap to roll the chicken around the spinach.
Secure the sides so the spinach is tucked neatly into the center. Remove the plastic and place the loaf on a sheet pan.
On a separate sheet pan toss the tomatoes with the other tablespoon of oil, balsamic vinegar, salt and pepper to taste.
In a preheated 350 degree oven add both the meatloaf and the tomatoes. Cook for 35 minutes. Slice and top with the roasted tomatoes.