Place top oven rack in the center position and pre-heat oven to 425°F. Line a large, rimmed baking sheet with parchment paper or a Silpat™ baking mat. Set aside.

Add the sliced beets and onions to a large bowl. Drizzle with 2 tablespoons olive oil and one-half tablespoon Italian seasoning. Season with salt and black pepper, to taste, and toss to combine.

Transfer seasoned vegetables to the baking sheet and spread into a single layer without overcrowding. Place in the pre-heated oven for 15 minutes.

While the vegetables are roasting, combine the Dijon mustard, honey, olive oil, lemon juice, and the remaining Italian seasoning in the same bowl used for the vegetables. Season with the garlic powder, plus some salt and black pepper, to taste, and stir to combine.

Add the pork tenderloin to the bowl and turn until thoroughly coated. Set aside.

Remove the sheet pan from the oven and turn the vegetables with a spatula, leaving room in the center for the seasoned pork tenderloin. Make sure the vegetables are still in a single layer and return to the oven for another 15-20 minutes, or until an instant read thermometer inserted into the pork reads at least 140°F.

Remove from oven and allow pork to rest for 5 minutes. Serve immediately with the roasted beets and onions. Enjoy!