Preheat oven to 450ºF with rack set in middle position. Line a baking sheet with parchment paper.

Rinse and dry radishes well. Trim and reserve greens. Halve any large radishes, small ones should stay intact.

In a mini food processor, combine butter, anchovies, ¼ cup packed reserved radish greens, chives, 2 teaspoons lemon zest, ½ teaspoon salt, and ½ teaspoon pepper.

Process until combined and mostly smooth, pausing to scrape down sides of bowl as needed. Measure out 1 tablespoon butter mixture and set aside.

In a large bowl, toss radishes with remaining butter mixture.

Transfer buttered radishes to prepared baking sheet and arrange in an even layer. The butter mixture will clump, so divide it as needed by topping each radish with a dollop.

Roast until tender and browned, 10–14 minutes.

Meanwhile, spread reserved butter on one side of both slices of bread. Shimmy radishes around to make room on baking sheet, then add bread, buttered side up, to sheet and continue to roast until bread is golden brown and crunchy, 12–15 minutes.