Preheat the oven to 375°F (190°C).
Prepare the onions, garlic, zucchini, yellow squash, eggplant, red bell pepper, and tomatoes, cutting the vegetables into small, relatively uniform pieces to encourage even roasting.
Arrange the prepared vegetables across a large baking sheet in an even layer.
Drizzle the vegetables with olive oil, then season them with dried oregano, crushed red pepper flakes, salt, and black pepper.
Toss thoroughly until the vegetables are evenly coated with the oil and seasonings. Redistribute them into a single layer on the baking sheet.
Roast for 25–30 minutes, stirring the vegetables once halfway through the cooking time. Continue roasting until they are tender and lightly golden.
While the vegetables are in the oven, place the quinoa in a fine-mesh sieve and rinse thoroughly under cold running water.
Pour 4 cups of water into a medium saucepan and bring it to a boil. Add the rinsed quinoa and stir to combine. A small amount of salt may also be added to the cooking water for additional seasoning.
Lower the heat, cover the saucepan, and simmer the quinoa for approximately 20 minutes, or until the liquid has been completely absorbed.
Remove the saucepan from the heat and keep it covered for several additional minutes. Uncover and gently separate the grains with a fork.
Transfer the cooked, fluffed quinoa to a large mixing bowl and add the roasted vegetables.
Fold the ingredients together gently until the roasted vegetables are evenly dispersed throughout the quinoa.
Add the pumpkin seeds or chopped walnuts, followed by the freshly squeezed lemon juice.
Taste the medley and adjust the seasoning with additional salt and black pepper as necessary. Mix until all ingredients are thoroughly incorporated.
Serve the roasted vegetable quinoa medley warm or at room temperature as a substantial vegetarian main course or a flavorful side dish.