How to Make It
In a small bowl, whisk the mustard with the vinegar. While whisking constantly, drizzle in 1/2 cup of the olive oil until emulsified. Season with salt and pepper.
Heat a griddle or large cast-iron skillet. Brush the tomatoes and fennel all over with the remaining 6 tablespoons of olive oil and season with salt and pepper. Working in batches, cook the tomatoes cut side down until lightly charred, about 3 minutes. Transfer to a platter charred side up. Working in batches, cook the fennel, turning once, until just tender and lightly charred, about 6 minutes. Transfer to the platter with the tomatoes. Drizzle the vegetables with the vinaigrette and garnish with the reserved fennel fronds and tarragon. Serve warm or at room temperature.