Hollandaise: Finely grate 3/4 teaspoon zest from grapefruit, then squeeze 1/4 cup juice; strain. Transfer zest and juice to a small saucepan with egg yolks, vinegar, salt, and cayenne. Whisk together continuously over medium heat until warmed through and slightly thickened, 2 to 3 minutes. (Do not let curdle; if mixture begins to heat too quickly, lower heat.) Slowly whisk in oil; continue whisking until thickened, about 1 minute. Remove from heat. Press plastic wrap directly on surface of hollandaise; set aside.

Salmon: Place grapefruit zest and juice in a large Dutch oven with broth, onion, fennel, salt, peppercorns, and 1 quart water. Bring to a simmer, then adjust heat as necessary to maintain broth temperature at 190 degrees on a thermometer. Cover and cook until fennel is tender, about 10 minutes.

Add fish to poaching liquid and simmer, adjusting heat as necessary to maintain broth temperature at 190 degrees on thermometer, until cooked through, about 5 minutes. Transfer salmon to serving plates alongside fennel, onion, and zest strips. Pour hollandaise over fish and garnish with cayenne. Serve with toasted baguette slices.