Preheat the oven to 390°F and lightly grease a 9×13-inch baking dish. Place the potato chunks in a large pot, cover with water, and add a pinch of salt. Boil for 10–12 minutes, until fork-tender but not falling apart. Drain well.
Heat olive oil in a large skillet over medium heat. Add the sliced sausage and cook for 5–7 minutes, turning occasionally, until lightly browned. Transfer to a plate and set aside.
Add the chopped onion to the same skillet and cook for 4–5 minutes until softened and lightly golden. Stir in the garlic and cook for 30 seconds.
Add the butter to the skillet. Once melted, whisk in the flour and cook for 1–2 minutes. Gradually pour in the chicken broth, whisking constantly until smooth and thickened. Stir in the Italian seasoning, smoked paprika, salt, and black pepper.
Arrange the cooked potatoes in the prepared baking dish. Scatter the browned sausage over the potatoes, then pour the onion gravy evenly over everything.
Sprinkle the shredded cheddar cheese evenly over the top. Bake uncovered for 20–25 minutes, until the cheese is melted and golden and the casserole is bubbling around the edges.
Remove the casserole from the oven and let it rest for 5 minutes. Garnish with fresh parsley and serve warm.
