Clean the leeks to remove any grit and sand, then slice into thin half moons (I like to clean the leeks after chopping them into thin slivers, but you can also clean, then chop).
Melt the butter in a heavy skillet over medium-low heat. When it's bubbling, add the chopped leeks, season with salt and pepper, and cook, stirring frequently, until soft and buttery, with minimal color, 8 to 10 minutes. Remove from the pan and set aside.
Meanwhile, chop the red onion and place in a small bowl. Add the sherry vinegar, a pinch of sugar, and a pinch of salt and stir to combine. Let the onions sit while the leeks cook.
In the same pan that you used to cook the leeks, heat 2 teaspoons of olive oil (or enough to coat the bottom of the pan) over medium heat. Turn the heat to medium-high, add the bread cubes and cook, stirring frequently, until crispy and brown.
When the leeks are cooked, the onion has been sitting for 10 to 15 minutes, and the croutons are ready, pour the can of tomatoes into a blender. Add the onion and any vinegar still in the bowl and blend until smooth.
Pour the soup into individual bowls and swirl the sautéed leeks and a generous dollop of pea pesto into each one. Top with croutons.
For the spring pea pesto (makes extra—halve if you don't want leftovers!):
To the bowl of the food processor, add all of the ingredients except for the olive oil. Process until you have a chunky paste.
With the processor running, drizzle in the olive oil until you have a smoother and emulsified mixture. Taste and adjust the salt, as needed (you probably won't have to add very much salt—the preserved lemon is plenty salty!).
A (former) student of English, a lover of raisins, a user of comma splices. My spirit animal is an eggplant. I'm probably the person who picked all of the cookie dough out of the cookie dough ice cream. For that, I'm sorry.