Preheat oven to 375°F and butter a 9x13 baking dish with 1 tablespoon of unsalted butter.
In a small saucepan over medium heat, melt 4 tablespoons butter with cream cheese, seasoned salt, and black pepper, whisking until smooth.
Whisk in half & half into the cream cheese mixture until thickened, then remove from heat to cool.
In a large pot, cover peeled potatoes with 2 inches of water and bring to a boil. Reduce to medium heat and cook potatoes until just tender, about 15-25 minutes.
Add broccoli florets to boiling potatoes during last 2 minutes of cooking, then remove broccoli with a slotted spoon and set aside.
Drain the potatoes in a colander and let cool enough to handle, then roughly chop into bite-sized pieces.
Return chopped potatoes to the empty pot to cool further and evaporate excess moisture.
Add the cooled sauce and sour cream to the potatoes and stir to coat evenly.
Gently fold the broccoli florets into the potato mixture and transfer everything to the prepared baking dish.
Sprinkle shredded Colby-Jack cheese evenly over the top.
Bake uncovered for 25-30 minutes until cheese melts and sauce bubbles.
Let casserole cool for 5 minutes before serving to allow sauce to set slightly.