Preheat your oven to 425°F and line one extra-large baking sheet or two standard baking sheets with parchment paper.
Add the diced Yukon Gold potatoes, sweet potatoes, broccolini, Brussels sprouts, orange bell pepper, red onion, and minced garlic to the prepared baking sheet. Drizzle with olive oil and season with garlic powder, onion powder, paprika, salt, pepper, and fresh thyme. Toss everything together until evenly coated, spreading the vegetables into an even layer. Transfer to the oven and roast for 20 minutes.
Remove the baking sheet from the oven and carefully transfer the broccolini to a plate. Add the sliced sausage to the baking sheet and toss everything together. Return to the oven and roast for another 25-30 minutes, until the potatoes are tender, the vegetables are caramelized, and the sausage is golden brown. Add the broccolini back to the baking sheet during the last 5 minutes of cooking to warm through.
While everything is roasting, prepare the creamy honey mustard sauce. In a small bowl, combine the grainy mustard, Dijon mustard, honey, Greek yogurt, olive oil, apple cider vinegar, and grated garlic. Whisk until smooth and creamy. Season with salt and pepper to taste.
Remove the sheet pan from the oven and allow everything to cool slightly. Drizzle generously with the creamy honey mustard sauce, reserving any extra for serving. Toss gently to combine or serve the sauce on the side for dipping. Enjoy!