About an hour prior to cooking, set out othe round roast to come to room temperature - this is essential to an accurate cooking time. The beef round will need to roast at 5 minutes per pound at high temperature. My numbers are based off a 2.25 lb beef round roast.
IF USING PEARL ONIONS: Blanch the pearl onions in boiling hot water for 30-60 seconds, then flash cool in a bowl of ice water. Trim off the roots and excess tips then pop the onion from the skin.
In a small bowl, stir together the flour, paprika, creamy horseradish, spicy brown mustard, rosemary and 1/4 cup olive oil until a thick paste forms. Rub the paste all over the beef until it spreads into an even layer. It might need to be worked a bit to spread evenly, but keep at it until it sitcks.
Fill the bottom of a roasting dish with the pearl onions, carrots, red peppers and garlic cloves. Drizzle the veggies with olive oil and season with salt and pepper to taste. Gently stir the veggies in the pan and move to the edges of the pan, leaving a well in the center. Place the seasoned beef round in the center of the roasting pan. Set aside.
Preheat the oven to 500 F | 260 C. While it preheats, slice the hassleback potatoes. Pour on the melted butter and season with salt and pepper. Lightly wrap in foil.
Place the potatoes on the top rack of the preheated over and the round roast, uncovered, on the middle rack. Cook the roast for 5 minutes per pound. At 2.25 pounds, I cooked mine at 500 F for 12 minutes.
Turn the oven off but DO NOT OPEN THE OVEN. Not for at leat 1.5 hours. Fight the temptation to check on it by making dressing for the salad.
Pour the apple cider vinegar, garlic, red pepper flakes, honey, spicy mustard, and olive oil into a mason jar. Shake until completely combined. Do not chill the dressing prior to serving. Wash and chop the salad and toppings, as desired.
Check the roast's temperature - for MEDIUM RARE: 140-145F | FOR WELL DONE: 145-150F. Remove from the oven along with the potatoes.
Turn the oven on to a broil on low setting (or move the rack to the center if yours does not have high/low settings). Gently open the slices of the potato and place a slice of cheese between each. Place under the broiler for 10-15 minutes, checking frequently, untilt the cheese is golden brown and bubbly.
Slice the roast beef with the roasted pearl onions and pan dripping.Serve alongside the cheesy potatoes. Dress the salad with your favorite toppings, or let the guests DIY their own salads and toppings! Add a dollop of Greek yogurt to the potato, (and chives and bacon for an overloaded version!)