Preheat oven to 400 degrees F.
Add potatoes to a saucepan and cover with water. Bring to a boil. Low boil potatoes until they are almost fork tender (about 10 minutes). Continue with vegetable prep as potatoes are cooking.
While potatoes are cooking, prepare other ingredients. Cut each corn on the cob into thirds (add to cooking potatoes near end of cooking, cut each onion into quarters, press garlic cloves through a garlic press, chop parsley, halve lemons, peel and de-vein shrimp (keeping tails) and cut sausage into 2" pieces.
Drain potatoes and corn.
In a small saucepan, melt butter on low heat, then add olive oil until heated. Stir in Old Bay Seasoning and garlic. Turn off heat.
In a large bowl add drained potatoes and corn, shrimp, sausage, onions, garlic, lemons, and butter/olive oil blend (everything except parsley). Mix well so everything is coated with oil and seasoning.
Pour bowl of ingredients onto a sheet pan. Spread out to make a single layer (or as close to a single layer as you can). Bake in oven until shrimp are opaque (10-15 minutes).
Remove from oven, garnish with chopped parsley and serve.