Step 1Cut the tops off the kohlrabi. Peel the kohlrabi and cut into 1-inch pieces. Place the kohlrabi in a 4-quart saucepan. Add enough water to cover by an inch, and heat over medium-high heat to boiling. Reduce the heat to a simmer and cook until the kohlrabi is fork-tender, about 20 minutes. Drain, reserving 2 cups of the cooking liquid; set aside.
Step 2Spray a large saucepan with nonstick cooking spray and add the butter. Heat until the butter melts and begins to sizzle. Add the shallots and cook until tender, about 1 to 2 minutes. Stir in the reserved kohlrabi broth and bring to a boil. Combine the flour and chicken broth in a small bowl until completely blended and smooth. Stir into the broth. Return to a boil and simmer until thickened, 1 to 2 minutes. Stir in the half-and-half, dill, tarragon and salt. Bring to a simmer. Stir in the kohlrabi and cook just until the kohlrabi is heated through and the liquid just begins to simmer. Do not overheat or the sauce may curdle. Pour into serving bowls and garnish with the sprigs of fresh dill.