Melt butter in a medium saucepan over medium-low heat. Add garlic and sauté until softened, about 2 minutes. Whisk in flour and cook until toasted and fragrant, about 3 minutes. Slowly add milk, whisking until smooth. Cook 1 minute until slightly thickened, then remove from heat. Whisk in 5 egg yolks (reserve remaining 2 for another use), one at a time, then cool, stirring occasionally.

Heat oven to 375°F. Butter a 6- to 8-cup soufflé mold, dust with bread crumbs, and set aside.Meanwhile, place spinach between 2 layers of paper towels, squeeze out excess liquid, and place in a large bowl. Once milk mixture has cooled, add to the bowl with spinach, along with cheese and salt, stirring to combine.

In a medium bowl beat egg whites until stiff. Fold ⅓ of whites into the spinach mixture. Gently fold in remaining whites until just combined. Transfer mixture to soufflé dish.

Bake until golden brown and puffy, about 35 minutes. Serve immediately.