Dirty rice is as common at the Cajun table as mashed potatoes and gravy is elsewhere. It's the meatiest, richest rice dish you'll ever eat, and it gets its color, its dirtiness, from glorious, glorious meat. The trick to this dish is getting a good char on the ground beef. I like to use ground sirloin, keeping it in a block and searing it like I would a steak before the meat is broken up and braised. That caramelized meat makes the difference between a good pot of dirty rice and something you'd be embarrassed to serve a Cajun grandmother. Just before you combine the meat mixture with the rice, you've basically got a dark roux chili. If you added some fresh tomato and cooked it down until it's nice and tight, you'd have a killer ragu for an incredible lasagna. You can make the meat part of the Dirty Rice ahead of time and freeze it for up to 6 weeks.
Sear the meat: Season the block of sirloin -- no fancy shaping needed, just use it how it comes out of the tray from the grocery store -- with 1 teaspoon of salt on each side.
In a large skillet, heat the oil over medium-high heat until it starts to smoke. Place the sirloin block in the skillet in one piece and let it sear until it really browns and caramelizes, 3 to 5 minutes. Then flip it and repeat, 3 to 5 minutes longer. Let it do its thing.
Once the block of sirloin is well seared, chop it up in the pan with a metal spatula to sear the inside bits. Add the black pepper, cumin, and cayenne and stir well. Cook for a minute. Add the beer to deglaze the pan, and cook 1 minute longer, scraping up any browned bits. Remove from the heat and set aside. At this point, you could freeze the meat.
Make the gravy: In a heavy Dutch oven over medium heat, make a dark roux using the oil and flour, about 45 minutes. Once it's the color of milk chocolate, add the onion, bell pepper, and celery and stir together. Cook for a minute. Stir in the garlic and cook for 1 minute longer. Add the beer and mix well. In 1/3-cup increments, add the stock, stirring well between each addition. Stir frequently, but not continuously, until you have a well emulsified gravy, thick enough to coat the back of a spoon.
Once the gravy is done, add the cooked beef. Add a splash of stock to the meat pan to deglaze to get the remaining "junk" out -- the delicious extra bits that stick to the pan -- and add to the gravy and meat. Bring the meat and gravy mixture back to a bare simmer. Cover and cook for 1 1/2 hours, or until the raw flour has all cooked out and the sauce has no chalky or floury flavor.
To serve: Add the cooked rice, butter, and green onions to the meat gravy in the pot. Stir it all together over low heat, just to warm it all through. Add salt to taste and serve.