FOR THE STEW:
Season the beef with a bit of sea salt and ground black pepper.
Toss the beef in all-purpose flour until coated. Set aside.
Heat oil in a skillet on moderate heat.
Add the garlic and shallots to the skillet; sautée until the garlic is fragrant and onions are translucent, approx. 2-3 mins.
Add the beef to the skillet and seer the exterior for approx. 4 minutes.
Add the carrots, mushrooms, tomato paste, fresh thyme, and 1 cup of Guinness.
Increase the heat to medium-high and bring to a simmer.
Simmer for approx. 5 minutes, stirring occasionally to make sure tomato paste breaks down.
Remove from heat and transfer to slow cooker.
Add the other cup of Guinness, and add the beef stock.
Cook on low for 3.5 - 4 hours.
At around the 2 hour mark, if you are finding that you want a thicker gravy, remove approx. 1 cup of juices from the slow cooker with a baster and into a mug or bowl.
Add 1-2 tablespoons of corn starch to the gravy, as well as 1/4 cup of cold water.
Whisk vigorously until the corn starch has dissolved.
Add it back to the slow cooker and stir.
FOR THE POTATOES:
Quarter the peeled potatoes and place them in a pot of water.
Bring to a boil and allow to boil for approx. 20 minutes or until the potatoes soften enough to break apart with a knife. (If you can stick a knife through the potato and it slides off when you lift, they are ready).
Drain the potatoes; make sure you get rid of any excess water.
Mash the potatoes slightly.
Add butter, milk and horseradish.
Continue to mash or whip with a hand mixer until you get the consistency you want.
Add more butter or milk if needed.