Cook 2 bunches jumbo asparagus (trimmed and peeled) in salted boiling water until crisp-tender, about 3 minutes; drain. Combine 2 egg yolks and 1 tablespoon each lemon juice and hot water in a heatproof bowl set over a saucepan of simmering water. Cook, whisking constantly, until thickened, 1 to 3 minutes (remove from the heat if the eggs start to scramble). Gradually whisk in 1 1/2 sticks cut-up butter until thickened, 3 to 5 minutes. Whisk in 4 teaspoons whole-grain mustard; season with salt and cayenne. Thin with lemon juice as needed. Serve over the asparagus.