Prepare a grill or grill pan for medium-high heat. If using an outdoor grill, heat over medium-high heat until a thermometer registers 500 degrees F. You want the grill or grill pan to be screaming hot so the watermelon chars quickly. If it takes too long it will turn mushy.
Cut the watermelon in half lengthwise. Cut each half into 1-inch-thick half-moon slices. Grill the watermelon slices, working in batches if using a grill pan, until just marked, about 2 minutes per side. Transfer to a colander placed over a large bowl to cool and collect the juices. Once cool enough to handle and you have about 1 tablespoon watermelon juice, slice the rind off the watermelon and cut the flesh into 1-inch cubes. Set aside.
Meanwhile, toss the beans with 2 tablespoons of the olive oil, 1/2 teaspoon salt and a few grinds of pepper and place onto the center of a large piece of foil so that most of the beans are flat but some are still overlapping. Roll up all 4 sides of the foil to create a tray, leaving the top open so you can still see the beans. Transfer the foil to the grill and cook, covered, until the beans are softened and charred, 7 to 8 minutes.
Whisk the vinegar, Dijon, shallot, reserved 1 tablespoon watermelon juice, 1/2 teaspoon salt and a few grinds of pepper in a small bowl. Whisking constantly, slowly drizzle in the remaining 3 tablespoons olive oil until emulsified.
Add the spinach, chives, tarragon, nutritional yeast and fennel with fronds to a large bowl. Fold in the watermelon, beans and dressing. Season with salt and pepper and serve immediately.