Preheat oven to 160C. For Port reduction, boil all the ingredients in a small saucepan over medium-high heat without stirring until syrupy and reduced to 2 tbsp (10-15 minutes). Remove thyme and peppercorns (discard) and pour reduction immediately into a blender.
Melt butter gently in a wide saucepan over low heat, add livers and turn occasionally until just opaque and half-cooked (5-10 minutes), then transfer to a blender with Port reduction and eggs, season well with sea salt flakes and blend until smooth. Add cream and pulse to combine, then strain through a fine sieve.
Pour liver mixture into a buttered 1.5-litre pâté dish, cover tightly with a lid or foil, place in a roasting pan with enough hot water to reach halfway up the sides and bake until pâté is just set (35-40 minutes). Set aside to cool, then refrigerate for 1 hour.
Meanwhile, for Port jelly, soften gelatine in 100 ml Port (3-5 minutes). Gently bring remaining Port to the simmer over medium heat, add gelatine mixture and stir to dissolve. Cool then pour over pâté and refrigerate for a further 1-2 hours until jelly and pâté are chilled and set.
For croûtes, preheat oven to 180C. Brush baguette slices with olive oil, then bake, turning occasionally, until golden and crisp (5-10 mins). Serve at room temperature with pâté.