Cut cauliflower into 1-inch florets. Add to medium pot w/ salted water. Cover and bring to a boil. Simmer until tender, about 15 minutes. Drain well and return cauliflower to pot. Using a potato masher or fork, mash cauliflower with 2 tablespoons each butter and milk. Season w/ salt and pepper. keep warm.
While cauliflower cooks, peel and finely chop onion. Trim stem ends from mushrooms, then thinly slice caps. In a liquid measuring cup, whisk together beef broth concentrate, mustard, mascarpone, and 1 cup water. Coarsely chop dill fronds and stems together.
Heat 1 Tbs oil in a medium nonstick skillet over med-high. Add onions and a pinch each of salt and pepper. Cook, stirring occasionally, until tender and lightly browned, about 5 minutes. Transfer to a medium bowl.
Add 1 Tbs oil, mushrooms, and a pinch each of salt and pepper to same skillet. Cook med-high heat, stirring occasionally, until golden brown, about 5 minutes. Transfer mushrooms to marscopone mix.
In bowl, mix cooked onions, beef, 1 tsp salt, 1 large egg, and a few grinds of pepper. Add breadcrumbs. Form mixture into 10 meatballs. Heat 1 tablespoon oil in same skillet over medium-high. Add meatballs and cook, turning once or twice, until browned but not cooked through, about 5 minutes. Pour off any fat from skillet.
Add mascarpone mixture and mushrooms to skillet with meatballs. Bring sauce to a simmer and cook over medium-high heat, basting meatballs with sauce, until sauce is thickened and meatballs are cooked through, about 10 minutes. Serve meatballs on top of cauliflower mash with mushrooms and sauce spooned over top. Sprinkle with chopped dill onions and a pinch each of salt and pepper.
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