Preheat the oven to 200°C (395°F).
Prepare the ingredients: peel and chop 1 large potato into chunks; peel and slice 1 red onion into wedges; cut 1 cup pumpkin or butternut squash into slices or chunks similar in size to the potato; slice ½ cup mushrooms; cut the tips off 4 cloves garlic and leave the peel on; have 4 sausages ready.
Place the potatoes, onion, pumpkin/squash, mushrooms, the 4 peeled-tip garlic cloves, and the 4 sausages on a single sheet pan in a single layer.
Drizzle 2 tablespoons olive oil over everything. Sprinkle 3 tablespoons fajita seasoning, 1 teaspoon dried oregano, and 1 teaspoon sea salt evenly over the pan. Toss the sausages and vegetables with your hands (or a spatula) until they are well coated with oil and seasonings.
Bake in the preheated oven for 20 minutes, turning/flipping the sausages and stirring/turning the vegetables once halfway through (about 10 minutes). Check doneness by inserting a fork into a piece of potato or squash—if it is tender, the bake is ready. If not tender, continue baking and check every 5 minutes until tender.
While the sheet pan cooks, make the pesto: in a food processor, combine 2 cups basil leaves, 3 tablespoons olive oil, 1 clove garlic, ¼ cup walnuts or pine nuts, and ½ teaspoon salt. Blitz until smooth. Taste and adjust (if desired).
When the vegetables and sausages are done, remove the sheet pan from the oven. Spoon or spread the pesto over the sausages and roasted vegetables on the pan, or serve the pesto on the side and toss individual portions with pesto to serve.
Serve the sausage and veggies with rice, buckwheat, quinoa, in sandwiches, or on their own.