In a large saucepan, sauté onion in olive oil until translucent and soft, about 5 minutes. Add in all the frozen vegetables and heat through, about 5 minutes. Add diced green peppers and mushrooms, and cook 5 minutes more. Add garlic and cook just until you can smell it. Season with salt and pepper to taste.
For the 'creamy' sauce (or substitute with a can of low-sodium cream of mushroom or celery soup): Move vegetables to the sides of the pan, leaving an empty space in the middle. Melt butter and mix in flour. Cook until it becomes a paste, about 1 to 2 minutes. Stir in milk and mix the sauce with the vegetables. Add vegetable stock, lemon pepper (if using) and oregano. Once the sauce starts to simmer, take it off the burner.
Preheat oven to 400°F. In a large bowl, make stuffing according to package instructions. Add whole-wheat croutons for extra crunch.
Place vegetable mixture on the bottom of a medium-size (6 cup/1 L) casserole dish. Sprinkle stuffing overtop and fluff with a fork (do not pack down). Bake for 15 minutes, or until the top starts to brown slightly. Remove from oven and let sit for a few minutes. Serve with a Greek or mixed-greens salad on the side, a great comfy meal any day of the week.