Season sirloin cubes generously with salt and pepper.
Place on a wire rack over a sheet pan, uncovered, and refrigerate 2 – 6 hours (or overnight).
Pat dry before searing.
Heat olive oil in Dutch oven over medium-high. Sear sirloin cubes in batches until deeply browned; remove.
Add garlic, shallot, thyme, and bay leaf. Sauté 2 – 3 minutes until fragrant.
Add carrot and celeriac. Sauté 5 – 6 minutes until beginning to soften.
Add tomato paste; cook 1 – 2 minutes until paste darkens and caramelizes.
Deglaze with wine; reduce by half.
Sprinkle flour; stir constantly 1 minute.
Add stock, Worcestershire, and parsley. Submerge the sachet (garlic, thyme, bay, toasted fennel seed tied in cheesecloth). Return sirloin to pot.
Bring to a simmer. Cover and cook on low 1½ – 2 hours until tender.
Remove sirloin and sachet; keep venison warm.
Simmer sauce uncovered 5 – 10 minutes if it needs reducing.
Swirl in cold butter and fennel pollen. Adjust seasoning.
Spoon mash, polenta, or gnocchi into warmed bowls. Arrange venison over the base.
Spoon sauce and vegetables over top. Garnish with parsley and black pepper.