First, prepare the breadcrumbs. Preheat the oven to 350ºF. Line a baking sheet with foil and set aside. Remove all the crust from the loaf. Cut it into cubes and pulse in a food processor until the crumbs reach the texture you want. Toss with some olive oil to lightly coat and pour onto the baking sheet, creating a thin, even layer of crumbs. Bake until golden brown, about 15 minutes. Remove from the oven and season with sea salt and pepper.
Combine the lemon juice, vinegar, olive oil, a generous pinch of sea salt, and some black pepper in a mason jar. Shake to combine. Set aside.
In a large bowl, combine the arugula, kale, feta, shallots, and 1/2 the breadcrumbs. Slowly drizzle a little vinaigrette on the salad and start mixing. Continue adding the vinaigrette until the leaves are coated. Taste. Season with sea salt and freshly ground black pepper if necessary.