It’s easiest to cut the meat when it’s very cold. Some people like to even put it in the freezer for 20 minutes or so. Using your sharpest knife, slice the meat into ⅛- to ¼-inch slices. Slice it again into strips ¼-inch wide, then dice those strips up. (No need to be too precise. It’s nice to have some texture here.) Once the meat is all diced up, put it in a bowl and back into the fridge to keep cold.
Prep and combine the remaining ingredients.
Remove the beef from the fridge and combine everything together in a bowl and mix.
Prepare bread by toasting it either in a pan with olive oil or butter or on a grill. Let cool. Cut some radish as thin as you can with a knife or mandolin.
Arrange the tartare on a plate. Top with radish and optional horseradish and trout roe. Drizzle a little extra virgin olive oil on top and serve with bread or crackers. Or enjoy it however you desire. Salt and vinegar chips work, too.