Start with a completely clean, dry mixing bowl and spoon. Any moisture can cause the seasoning mix to clump or spoil more quickly.
Add the kosher salt, black pepper, onion powder, garlic powder, smoked paprika, dried thyme, dried rosemary, dried oregano, ground mustard, brown sugar, crushed red pepper flakes (if using), and celery seed (if using) to the bowl.
Use the back of the spoon or your fingers to crush any clumps of brown sugar, paprika, or garlic powder so the blend is as smooth and even as possible.
Stir until the seasoning is fully combined. The color should be evenly reddish-brown with small flecks of herbs throughout and no visible pockets of plain salt.
Taste a tiny pinch of the seasoning. It will be salty and concentrated, which is normal. Adjust to your liking—add more garlic powder for a stronger garlic flavor, more thyme for herbal notes, or more red pepper flakes for heat.
Transfer the finished seasoning mix to a small glass jar or spice container with a tight-fitting lid. Label the jar with the name and date.
Store the jar in a cool, dark cabinet away from heat and direct sunlight. For best flavor, use within 6 months.
For a 2.5–3 lb beef chuck roast, use about 2–2 1/2 tablespoons of seasoning. For a 4–5 lb roast, use about 3–3 1/2 tablespoons. Pat the roast dry, sprinkle the seasoning generously on all sides, press it into the meat, then sear in a hot skillet with a little oil before cooking in a slow cooker, pressure cooker, or oven until fork-tender.