How to Make It
Preheat the oven to 375°. In a large bowl, coat the bacon with the brown sugar and 1/2 teaspoon of pepper. Arrange the bacon in an even layer on a parchment paper–lined baking sheet. Top with another piece of parchment paper and another large baking sheet. Bake for about 25 minutes, until the bacon is lightly browned and the fat is rendered. Remove the top baking sheet and parchment paper. Bake the bacon for about 15 minutes longer, until well browned and nearly crisp. Using tongs, transfer the bacon to a rack to cool and crisp, then coarsely chop.
Meanwhile, in a large skillet, heat 1 tablespoon of the oil. Add the fennel and a generous pinch each of salt and pepper. Cook over moderately high heat, stirring occasionally, until just softened and lightly browned, about 4 minutes. Transfer to a plate and let cool.
In a large serving bowl, whisk the mustard with the lemon juice and vinegar. Gradually whisk in the remaining 1/3 cup of oil until emulsified. Season the dressing with salt and pepper. Add the fennel, spinach and basil to the bowl and toss well. Season with salt and pepper and toss again. Sprinkle the feta and candied bacon on top and serve right away.