How to Make It
In a saucepan of salted boiling water, blanch the 4 cups of parsley until bright green, 45 seconds. Drain and immediately transfer to an ice bath to cool. Drain well and transfer to a food processor. Add the dill, garlic, 1 cup of water and 2 1/2 tablespoons of the lemon juice and puree until nearly smooth. With the machine on, add the 1/2 cup of olive oil until smooth. Season the sauce with salt and pepper and transfer to a bowl.
Light a grill and brush the grate with olive oil. Brush the onion wedges with oil and season with salt and pepper. Grill over moderate heat, turning once, until lightly charred and softened, 7 minutes total. Transfer the onions to a bowl and add the lemon zest, thyme and remaining 1 tablespoon of lemon juice and toss.
Grill the bratwursts over moderately low heat, turning, until an instant-read thermometer inserted into each sausage registers 165°, 15 minutes. Spoon some of the parsley sauce onto a platter and top with the bratwursts and onions. Garnish with parsley and serve with mustard and Torn Garlic Bread, passing the rest of the parsley sauce at the table.