Season tenderloin roast with coarse sea salt and freshly ground black pepper.
Slow cook roast for 80-90 minutes (or until internal temperature reaches 125°F for rare). This can vary greatly from roast to roast, please measure. Remove roast from oven.
Right before removing, heat a large cast iron pan or other skillet. Add oil.
Add tenderloin, turning occasionally until evenly browned. This only takes 3-4 minutes.
Low heat and add butter, allowing it to melt fast. Remove from heat and spoon hot butter over roast.
Remove, tent with foil and allow to rest while you make the blueberry sauce. If you made your blueberry sauce ahead of time, allow to rest for about 5 minutes. Not as much time is needed for a reverse sear.
Using browned and beef seasoned butter already in the pan, add shallots and allow to soften for 1-2 minutes.
Add remaining ingredients including red wine, fresh blueberries, thyme sprigs, Iight brown sugar and balsamic vinegar..
Bring to a low simmer, stirring every so often. Heat until blueberries start to burst and reduce in size. Sauce should start to thick and become syrupy, about 5-10 minutes. Be mindful that it will continue to thicken as it cools, so don’t wait until it is super sticky to remove from heat.
Remove from heat and allow to cool for 2-4 minutes. Remove thyme sprigs.
Serve on the side of beef or ladle over beef tenderloin on serving platter.
Thinly slice beef tenderloin and place on a serving platter. Top with toasted hazelnuts and flakey sea salt, such as Maldon sea salt.
You can serve the sauce on the side with other sauces (such as horseradish) as well or ladle over the top for a stunning centerpiece.
You can use frozen blueberries as well, just thaw them and drain well.You can prepare the sauce ahead of time if you don't want to do it after searing. Simply start in a small saucepan with 2 tablespoons unsalted butter, follow the directions and then correct seasoning with salt and pepper at the end. To toast hazelnuts: place them in a dry skillet over medium heat, tossing for about 3-4 minutes. Toasting brings out natural oils, flavor and aroma.You can use dried thyme instead of fresh. Use about ½ teaspoon.