Boil the potatoes. Add the chopped potatoes to a pot and cover with cold water with 1 tablespoon of kosher salt. Bring to a gentle simmer until the potatoes are very tender.
Saute the bacon and brussels. While the potatoes cook, heat a 3 qt braiser over medium-low heat. Add the bacon and cook for 8-10 minutes, stirring occasionally, until the bacon is crispy and the fat has rendered. Use a slotted spoon to transfer the bacon to a bowl. Add the sliced yellow onion, sliced garlic, and shredded brussels to the skillet. Cook for 4-5 minutes, stirring often, until the brussels are wilted and bright green. Transfer to the bowl with the bacon and set aside.
Make the colcannon. Drain the potatoes and return them to the pot they were boiled in. Add the butter and heavy cream and mash until smooth. Fold in the bacon/brussels mixture and season to taste with salt and pepper.
Preheat an oven to 400 F.
Brown the meat and veggies. In the same braiser, heat the olive oil over medium heat. Add the ground sirloin and lamb and cook until browned, breaking the meat up into small pieces as it cooks. Add the diced onion, carrot, celery, and minced garlic and cook for 4-5 minutes or until the vegetables are soft.
Finish the filling. Stir in the tomato paste, Worcestershire, and better than bouillon paste and cook for 2 minutes. Sprinkle in the flour and cook for another 2-3 minutes, stirring constantly, to cook out the raw flour taste. Stir in the beef stock and bring to a simmer for 3-4 minutes, or until the filling is thick. Turn off the heat, stir in the peas, and season with salt and pepper to taste.
Top and bake. Remove the pan from the heat and spread the mashed potatoes in an even layer over the top. Bake for 20-25 minutes, until the filling is bubbling and mashed potatoes are golden brown on top. Rest for 10 minutes before serving.