Preheat oven to 300 degrees.
Rinse the corned beef thoroughly. It will be pretty darned salty. You can discard the seasoning packet it comes with. Not needed for this recipe. No way! Pat it dry and set aside.
Heat a Dutch Oven or a wide-large pot to medium heat. Add olive oil and heat.
Add peppers, celery and onion. Cook about 6 minutes to soften, stirring here and there.
Add garlic and cook another minute.
Wisk together the Guinness with the mustard and honey.
Add corned beef to the Dutch Oven along with the beer mixture. Bring to a quick boil then reduce heat to simmer about 5 minutes. Cover and transfer to the oven.
Cook for 4 hours, flipping the corned beef once about half way through.
Remove from heat. Stick a fork in the corned beef and test for tenderness. You should be able to pop the fork right in there. If it isn’t tender enough for you, bake it another 30 minutes or so. It should be SUPER TENDER at this point.
Remove the beef and rest a few minutes while you make your gravy.
For the gravy, strain out the liquids into a separate bowl. You should have about 1 cup. Discard the solids.
Heat the empty Dutch Oven back to medium heat on the stove and add butter. Melt.
Swirl in the flour and stir to form a roux. Cook a couple minutes to brown nicely. Add the strained juices from the corned beef and reduce heat to a simmer. Stir often to thicken the gravy. It is SO GOOD!
Slice the corned beef against the grain and serve over mashed potatoes, then top with your spicy Guinness gravy, and Caramelized Cabbage on the side. Here is a Cheesy Garlic Mashed Potatoes Recipe to consider. Or just use your favorite recipe.