Blanch and peel pearl onions (or thaw frozen). Pat dry.
Brown mushrooms in 3+ batches in hot oiled pot until deeply golden. Season. Set aside.
In same pot, cook pearl onions in olive oil 8-10 minutes until golden. Add garlic and whole spices. Cook 2 minutes.
Add tomato paste. Cook 1 minute. Pour in red wine. Reduce by half, 5-7 minutes.
Add plum tomatoes, stock, vinegar, and sugar. Season.
Return mushrooms to pot. Stir gently.
Bring to simmer. Cover. Cook on lowest heat 45-55 minutes. Remove lid last 10 minutes.
Remove whole spices. Adjust seasoning. Serve with bread or pasta, scattered with parsley.
Notes
This stew is significantly better the next day. The spices integrate fully and the sauce deepens overnight. Reheat gently with a small splash of water if needed.