Add the zucchini, yellow squash, onion and bell peppers to a large mixing bowl and toss with the olive oil, oregano, thyme, salt, pepper and garlic until coated. Divide the vegetables between 2 baking sheets and arrange them in a single layer. Roast, turning the vegetables halfway through, until they start to deepen in color and are slightly tender, about 10 minutes.
Add the roasted vegetables to the bowl of a slow cooker along with the stock, stewed tomatoes, cream and bay leaf. Set the slow cooker to low and cook for 3 hours.
Remove the bay leaf, add the lemon juice and, using an immersion blender, pulse to blend the soup until you achieve a smooth and rustic texture. Stir, taste and adjust the seasoning as needed.
Serve in a bowl garnished with the chopped parsley. If freezing, cool the mixture completely, then transfer to freezer-safe bags, seal and place in freezer.