Preheat oven to 450 degrees. In a medium bowl, combine bread with 1 tablespoon oil; season with salt. Set aside. In a medium pot, heat 1 1/2 teaspoons oil over medium-high. Add spinach; season with salt. Cook, stirring, until wilted, 2 minutes. Transfer to a colander, let cool, then squeeze out water. Coarsely chop.
Wipe pot clean and heat 1 1/2 teaspoons oil over medium. Add onion, garlic; season with salt and pepper. Cook, stirring occasionally, until onion is soft, about 8 minutes.
Add wine to pot and cook until almost evaporated, about 5 minutes. Add flour and cook, stirring, 30 seconds. Gradually whisk in half-and-half. Add lemon juice and bring mixture to a boil over high.
Remove from heat and stir in chopped spinach and chicken; season with salt and pepper. Transfer to a 2-quart baking dish and top with bread pieces. Bake until bread is golden brown and mixture is bubbling, 8 to 10 minutes.
Assemble and refrigerate this, uncooked, a day ahead. Add bread before baking.