For the brisket: Add the olive oil to a large Dutch oven over medium-high heat. Sprinkle the brisket with Miss Brown's House Seasoning. Sear the brisket fat-side down until a deep golden crust forms, 5 to 7 minutes. Flip and sear other side until deep golden brown, another 5 minutes. Remove to a sheet tray.
Add the onions and bay leaves and season with salt and pepper. Stir until coated in fat. Add the beer and bring to a boil. Put the brisket on top of the onions, cover and place in the oven for 3 hours.
For the BBQ sauce: While the brisket cooks, add the vinegar, brown sugar, brown mustard, whole-grain mustard, yellow mustard, onion, garlic, bourbon and cayenne to a medium saucepan over medium-high heat. Bring to a boil and cook until the onion becomes slightly translucent, about 5 minutes. Remove from the heat, add to a blender (or use an immersion blender) and puree until smooth, being careful of hot steam. Pour back into the pan and simmer over medium-low until slightly thickened and reduced by 1/4, about 30 minutes.
At the 3-hour mark, pour 1 cup of the BBQ sauce over the brisket, return to the oven and cook until the sauce has melted into the brisket, 30 minutes more. Use a knife to test the tenderness of the brisket. It should pierce the brisket very smoothly, with no pushback.
Remove the brisket from the pot and slice against the grain. Discard the bay leaves. Serve with sliced white bread and all those slow-braised beer onions. Don't forget about the extra BBQ sauce for dipping. Store any extra sauce in an airtight container in the refrigerator for up to 2 weeks.
Miss Brown's House Seasoning:
Stir together the onion powder, paprika, garlic powder, salt and pepper in a medium bowl. Keep in an airtight container. Makes 5 teaspoons.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.