Slowly poaching the ahi tuna in olive oil at a low temperature results in extremely tender, rich fish. Think of it as a homemade alternative to canned tuna, the “chicken of the sea”.
Season the tuna on both sides with salt and pepper and let sit for 20 minutes.
Add to an oven safe baking dish or Dutch oven and cover with the olive oil, using more if needed so the fish is just covered. Add the shallots and rosemary then cover and bake until the fish is tender and just cooked through, about 30 minutes.
Let cool to the touch, about 15 minutes and discard the rosemary. Add the fish and shallots to a large mixing bowl and save the oil for another use. Add the mayonnaise and vinegar, using a fork to mash up the tuna. Season with salt and pepper then set aside until ready to use.
The tuna mixture can be made a day ahead of time then covered and refrigerated until ready to use.
For serving:
Char the red pepper over gas flame or in broiler until blackened on all sides. Enclose in a paper bag for 10 minutes.
After roasting the pepper, carefully peel off the charred skin. Slice the top part off the pepper discarding the stem and seeds. Cut the red pepper into strips and set aside.
Toast the sourdough bread then top 2 of the slices with tuna salad, avocado slices, cucumber slices, roasted red pepper, pickled onions and sprouts.
Close both sandwiches and carefully slice in half with a bread knife. Serve immediately or wrap in parchment paper and refrigerate until ready to serve.