Preheat the oven to 375°F.
On a baking sheet, toss the bread cubes with the oil, garlic, fennel seeds, caraway seeds, and salt and toast until crisp, about 8 minutes.
In a large stock pot, cover the potatoes with cold, salted water and bring to a simmer. Cook until soft but not mushy, about 16 to 20 minutes. Drain the potatoes and return to the pot. Add the butter, milk, buttermilk, salt and pepper. Mash until smooth. Season with salt and pepper to taste.
Preheat the grill to medium-high or heat a grill pan over a medium-high heat.
Combine all of the pastrami rub spices in a shallow dish and dredge the steaks in the spice rub, coating all sides. Grill to your desired doneness (4 to 5 minutes per side for medium-rare, or 135°F internal temperature). Rest for 5 minutes before serving
In a small bowl, combine the horseradish, garlic, salt and 2 tablespoons sour cream. When fully combined, gently fold the remaining sour cream and chives into the mixture. Season to taste.
Add all the vinaigrette ingredients to a jar with a lid and shake until emulsified. Season to taste.
In a large bowl, toss together the beets, fennel, rye croutons, parsley and mustard vinaigrette.
Place the steaks on serving plates with mashed potatoes, beet salad and horseradish cream.