Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish with non-stick spray or butter.
Place the cubed potatoes into a large Ziploc bag or a large mixing bowl. Pour in the melted butter, minced garlic, dried parsley, 1/2 teaspoon of salt, and 1/4 teaspoon of pepper.
Seal the bag and shake vigorously (or toss in the bowl) until the potatoes are evenly coated with the butter and seasoning mixture.
Spread the seasoned potatoes into the prepared baking dish in a single layer. Bake for 45-50 minutes, or until the potatoes are fork-tender and starting to brown.
While the potatoes are baking, heat the olive oil in a skillet over medium-high heat. Add the sliced smoked beef sausage and onions. Fry for 8-10 minutes until the sausage is browned and onions are soft and caramelized.
Remove the baking dish from the oven. Carefully stir the cooked sausage and onion mixture into the potatoes.
Sprinkle the shredded cheddar cheese evenly over the top of the casserole.
Return the dish to the oven and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Garnish with extra parsley if desired and serve hot.