In a large pot, combine water, kosher salt, brown sugar, peppercorns, garlic, bay leaves, thyme, onion, and lemon slices. Bring the mixture to a simmer over medium heat, stirring occasionally until the salt and sugar are completely dissolved. Let it cool to room temperature.
Once the brine has cooled, place your beef roast in a large, sturdy plastic bag or non-reactive container. Pour the brine over the beef roast, ensuring that it is completely submerged. If needed, you can weigh the roast down with a plate to keep it submerged.
Seal the bag or cover the container and place it in the refrigerator. Let the beef roast brine for at least 12 hours or up to 24 hours for best results.
After brining, remove the beef roast from the brine and rinse it thoroughly under cold water to remove any excess salt. Pat it dry with paper towels.
Now you can cook your beef roast using your preferred method, whether it's roasting, grilling, or slow cooking. The brining process will help to infuse the meat with flavor and keep it juicy and tender.