Preheat oven to 425 degrees F. Coat baking sheet with cooking spray.
Heat large, deep skillet over medium heat. Add half the oil and swirl to coat pan. Add onion and cook 5 minutes, or until onion becomes translucent, stirring often. Stir in white mushrooms, garlic and salt, and cook 8 to 10 minutes, or until mushrooms are soft and most of the liquid has evaporated, stirring frequently. Stir in rice, and cook 1 to 2 minutes longer, or until heated through. Remove from heat and stir in goat cheese.
Fill each portobello with about 1/3 cup filling mixture. Toss tomatoes with lemon juice in a bowl, and lay tomato halves cut-side up over tops of filled portobellos.
Place stuffed portobellos on baking sheet greased with the remaining olive oil, and sprinkle each with 1 teaspoons breadcrumbs. Bake 30 to 35 minutes, or until mushrooms are cooked through and breadcrumbs are golden. Sprinkle wuth chopped parsley.