How to Make It
If using regular bunch spinach, rinse and stem leaves, then chop into 2- to 3-in. pieces. Heat oil in a 6- to 8-qt. pot over medium heat. Add onions and garlic; cook, covered, until translucent, 5 to 7 minutes. Add 1/2 cup water, a pinch of salt, and enough spinach to fill pot. Cook, covered, just until wilted; add rest of spinach and turn to wilt. (If using frozen spinach, add now.) Cook, covered, just until evenly wilted and tender (add some water if necessary to keep steam going). Season with salt to taste.
Drain spinach; return to pot. Stir in béchamel, parmesan, horseradish, and sour cream.
Heat over low heat until bubbling, then spoon into a serving dish. Top with toasted bread crumbs and bacon.
*At Alfred's, they use Bloomsdale spinach, a small-leafed, tender heirloom variety that needs no chopping or stemming. To make toasted bread crumbs, whirl crusts of brioche or other soft white bread in a food processor into crumbs, then toast at 350° until golden, 10 minutes.
Make ahead: Bread crumbs, up to 1 week, cooled and stored airtight at room temperature. Bacon, up to 4 days, chilled airtight. Spinach through step 2, up to 2 days, chilled (reheat with a bit of milk to loosen).