How to Make It
Bring a large saucepan of water to a boil and fill a large bowl with ice water. Blanch the fava beans in the boiling water for 1 minute. Drain and transfer to the ice water to cool; drain again. Using kitchen shears, snip off a small bit of skin at one end of each fava and pinch out the beans. Discard the skins.
Preheat the broiler. In a medium ovenproof skillet, melt the butter. Pour half of the melted butter into a small bowl and stir in the panko. Add the onion to the skillet and cook over moderately low heat, stirring occasionally, until softened, about 5 minutes. Add the zucchini, season with salt and pepper and cook over moderately high heat until softened and lightly browned, about 5 minutes. Stir in the cream, tarragon and fava beans and simmer over moderate heat until the cream is slightly thickened, about 5 minutes longer.
Broil the vegetables 8 inches from the heat until the crumbs are toasted, about 2 minutes.