Preheat the oven to 400°. Set the garlic cut side up on a large square of foil and top with 3 thyme sprigs, the rosemary and the olive oil. Wrap tightly in foil and roast for about 1 hour, or until softened and caramelized. Let cool in the foil. Squeeze the softened garlic cloves from the skins into a bowl and mash with a fork.
In a food processor, pulse the trout until finely chopped. Add the cream cheese, half-and-half and lemon zest and process until combined. Add to the roasted garlic, along with the minced thyme and pepper. Garnish with the remaining 2 thyme sprigs.
The dip can be refrigerated for up to 4 days; let return to room temperature before serving.