Remove rump roast from its net and reserve net for later. Place the rump roast in a resealable plastic bag with balsamic vinegar, squeeze out the air, seal, and refrigerate for 30 minutes.
In a food processor, pulse together walnuts, 3 tablespoons matzo meal, garlic, red pepper flakes and 1 teaspoon salt to the consistency of coarse cornmeal. Add olive oil while blending to make a paste; set aside.
Preheat oven to 450° F.
Remove rump roast from bag, pat dry and lay flat on a cutting board. Layer with walnut paste,
arugula and sun dried tomatoes. Roll and return roast to its net. Season the exterior evenly with remaining 3 tablespoons matzo meal, 1 teaspoon salt, rosemary, and pepper. Set in roasting pan with a rack, uncovered.
Roast for ten minutes. Reduce heat to 325° F and roast an additional 60 minutes for medium rare.
Allow roast to rest 10 minutes before slicing.