20 gmsea salt2 tbspfinely chopped thyme, plus 4 sprigsFinelygrated rind of 1 lemon, plus 4 thin lemon slices1garlic clove, finely chopped, plus 2 extra garlic cloves, bruised400 gmsashimi-grade tuna, cut into 3cm x 3cm pieces750 ml (3 cups)olive oil2fresh bay leaves2small red chillies, halved lengthways6black peppercorns1 loafolive bread, thinly sliced50 mlextra-virgin olive oilTo serve:lemon wedgesWhite bean purée200 gmdried white beans, soaked overnight in cold water, drained1onion, coarsely chopped1garlic clove, finely chopped70 mlolive oil70 mllemon juice
Combine salt, chopped thyme, lemon rind and chopped garlic in a bowl, season to taste with freshly ground pepper and mix well. Place tuna pieces in a non-reactive container, scatter with salt mixture, cover and refrigerate to lightly cure (10-15 minutes).
Meanwhile, combine olive oil, thyme sprigs, lemon slices, bay leaves, chilli and peppercorns in a wide saucepan large enough to hold tuna in a single layer. Heat over low heat, bringing slowly to 120C (12-15 minutes). Brush salt mixture off tuna, rinse and pat dry with absorbent paper. Add tuna to oil, ensuring it is completely covered (add more oil if necessary) and cook, maintaining temperature for 10 minutes. Cool tuna in oil, then remove with a slotted spoon and transfer to a sterile jar. Pour over oil (make sure no liquid given off from the fish goes into the jar), and refrigerate until required. Preserved tuna will keep refrigerated for 1 week.
For white bean purée, combine beans in a saucepan with onion, garlic and plenty of cold water, bring to the boil over medium-high heat and cook until very tender (30-40 minutes). Drain, transfer to a food processor and process until smooth. With motor running, add oil in a steady stream, process until thick and amalgamated, then add lemon juice and process to combine. Season to taste and refrigerate until required.
Meanwhile, preheat oven to 180C. Place bread slices in a single layer on an oven tray, drizzle each side with olive oil and season to taste. Bake until crisp and golden (10-15 minutes), then, when cool enough to handle, rub with bruised garlic cloves. Cool completely. Crostini will keep in an airtight container for 2 days. Serve crostini with white bean purée and preserved tuna, with lemon wedges for squeezing.