Preheat oven to 450°F.
Place potatoes in a large sauce pot, cover with water by at least 1 to 2 inches and place over high heat to bring up to a boil. Once boiling, add some salt and cook until potatoes are tender, about 10-12 minutes.
While potatoes are cooking, trim silver skin or connective tissue off tenderloins with a very sharp thin knife.
Place them on a non-stick cookie sheet with a rim. Coat tenderloins with EVOO then sprinkle with the grill seasoning. Transfer to oven and roast for 25-30 minutes, until the internal temperature reaches 145°F.
Place green beans and half the chopped onion onto a cookie sheet, drizzle with a little EVOO and season with some salt and pepper.
Toss to coat evenly then transfer to the oven with the pork to roast for the last 12-15 minutes the tenderloins are in the oven. Give them a shake about 5 minutes into the roasting time.
Let meat rest while you finish the potatoes and sauce.
Once you have the beans working in the oven, start the BBQ pan gravy: Heat a medium skillet with 1 turn of the pan of EVOO, about 1 tablespoon, over medium-high heat. Add the remaining onion, garlic and a little salt and pepper to the pot. Cook, stirring frequently, for 2-3 minutes or until the onions start to get tender. Add 1 tablespoon of butter and melt. Add the flour and cook for 1 minute.
Add BBQ sauce and whisk in the chicken stock. Bring up to a bubble then turn down the heat and let the pan sauce cook until thick.
Once the potatoes are tender, drain and return them to the pot. Add 1/2 cup of milk and 3 tablespoons of butter, season with salt and pepper, and add the chopped green onions. Smash with a potato masher or back of a fork to desired consistency.
To serve, thinly slice 2 of your roasted pork tenderloins, reserve the other 2 in the fridge for the next day.
Platter it up drizzled with your BBQ pan sauce, and serve alongside the green onion smashed potatoes and the roasted string beans.