Lay the bacon in a single layer on a rimmed baking sheet. Bake until the crispy, 12 to 13 minutes. Transfer the bacon to a cutting board and chop; set aside. Carefully pour the bacon fat into a small heatproof bowl and measure 2 tablespoons of the fat into a small saucepan. Discard any remaining bacon fat or reserve for another use.
Meanwhile, soak the onion in a medium bowl of cold water for 15 minutes, then drain and transfer to a large bowl. Add the spinach, carrots and sliced fennel and let sit at room temperature.
Add the ginger and garlic to the saucepan with the bacon fat and cook over medium heat, stirring occasionally, until softened and fragrant, about 1 minute. Add the miso and cook, whisking, until smooth and just warm. Add the vinegar, honey and sesame oil and cook, whisking frequently, until just simmering and smooth, about 2 minutes.
Add the bacon to the bowl with the spinach then pour in the warm dressing. Toss gently until the spinach just begins to wilt. Transfer the salad to a large serving bowl and garnish with the fennel fronds and sesame seeds.