Preheat the oven to 425 degrees F. Lightly oil a baking sheet or coat with nonstick cooking spray and set aside. In a large pot of boiling salted water, cook the potatoes until just tender, about 10-13 minutes. Add in the corn pieces during the last 5-8 minutes of cooking time. Drain both the potatoes and corn and set aside.
In a small bowl, combine the butter, olive oil and Old Bay Seasoning and season the mixture with salt and pepper. Place the potatoes, corn, shrimp, clams, (pre-cooked) sausage rounds and lemons in a single layer onto the prepared baking sheet. Stir in the butter mixture and gently toss to combine.
Place the sheet tray into oven and bake for 12-15 minutes, or until all the clams have opened and the shrimp are opaque and pink. Garnish with fresh parsley leaves and a squeeze of lemon juice and serve.