Step 1

Stir together the onions, vinegar and sugar in a small bowl or jar and season with a teaspoon of salt and some pepper, to taste. Set aside to pickle for at least 20 minutes or preferably cover and transfer to the fridge for a few hours or up to 2 weeks.

Step 2

Meanwhile, bring a large pot of salted water to a boil over high heat. Cook the pasta according to the package directions then strain and set aside to cool to room temperature.

Step 3

Whisk together the mayonnaise, pesto, Dijon, lemon zest and half of the juice of the lemon in a large bowl. Scoop about 2 tablespoons of the onion pickling liquid into the bowl and whisk to combine. Season to taste with salt and pepper. If desired, add the fennel to a bowl and toss with the remaining lemon juice to pull back on the strong fennel-y flavor.

Step 4

Add in the pasta, sliced fennel and onions, leaving the pickling liquid behind. Reserve this pickling liquid to add to salad dressings or simply add some more sliced onions into the mix so that you have more pickled onions on hand for future recipes. Add in the tuna and toss well to combine.

Step 5

Gently stir through the parsley, basil and fennel fronds and season with a bit more salt, pepper and lemon juice, if needed.

Step 6

Serve immediately or store in the fridge for up to 2 days.