Preheat oven to 375°. Cut pork tenderloin down the middle lengthwise, about halfway deep. Open the pork loin like a book (cut deeper if needed to help it unfold but be careful not to cut all the way through). Cover with plastic wrap and pound flat with a meat mallet or sturdy saucepan to about ¼-inch thick. Set aside.
Heat 1 tbsp olive oil in a large skillet over medium heat. When hot add garlic and cook briefly until fragrant. Add kale, mushrooms, ½ tsp salt and ¼ tsp pepper. Cook, stirring occasionally, until chard is wilted and mushrooms are tender and browned, about 7-10 minutes.
While chard and mushrooms are cooking, spread goat cheese onto the pork loin.
When mushroom mixture is done cooking, add lemon juice and mix to combine. Remove from heat and scatter over the top of the goat cheese.
Starting from a long edge, roll pork loin into a tight cylinder. Tie up with kitchen twine at about 2-inch intervals.
On a large sheet pan combine broccoli, cauliflower, or Brussels sprouts, with carrots or beets on the sheet pan. Toss with 1 tbsp olive oil, ½ tsp salt and ¼ tsp pepper.
Add pork tenderloin to the sheet pan seem-side down. Bake for 25-30 minutes, or until 145° internal temperature (be sure to measure with the thermometer horizontal, into the thickest part of the meat, not the filling). Remove pork to a cutting board to rest and return vegetables to oven, if needed, until browned.
While pork tenderloin is roasting, heat a large pot of salted water over high heat. When boiling add potatoes and cook for 15-20 minutes until very tender. Drain and return to pot. Mash potatoes to desired consistency. Stir in butter, 1/4 cup of milk, ½ tsp salt, and ¼ tsp pepper. Add more milk as needed and adjust seasoning with salt and pepper to taste.
Allow pork tenderloin to cool for 5-10 minutes before slicing. Serve with vegetables and mashed potatoes.