Preheat oven to 450 degrees Fahrenheit. In a medium bowl, combine bread with 1 tablespoon oil; season with salt. Set aside.
In a medium pot, heat 1 ½ teaspoons oil over medium-high heat. Add spinach; season with salt. Cook, stirring, until wilted, about 2 minutes.
Transfer to a colander, let cool, then squeeze out water.
Coarsely chop spinach and set aside.
Wipe pot clean and heat 1 ½ teaspoons oil over medium heat. Add onion and garlic, then 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 the sauce 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.
