In a large saucepan cook and stir onion and garlic in hot oil about 3 minutes or until tender. Add broth; bring to boiling. Add broccoli. Return to boiling; reduce heat. Cover and simmer for 8 to 10 minutes or until tender. Transfer vegetables to a serving platter, reserving broth in pan (add additional broth, if necessary, to measure 1/2 cup).
In a small bowl combine lemon juice and cornstarch; add to broth in saucepan. Cook and stir until thickened and bubbly; cook and stir for 2 minutes more. Stir in dill. Season to taste with kosher salt and pepper. Spoon sauce over vegetables; toss to coat. If desired, garnish with lemon slices.