Consider when preparing that time is needed to chill. Adapted from "Cooking at Home With Bridget and Julia," by Bridget Lancaster, Julia Collin Davison and the editors at America's Test Kitchen (America's Test Kitchen, 2017).
Heat the tablespoon of oil from the sun-dried tomatoes in a large saucepan over medium heat. Once the oil shimmers, add the green onions and cook until softened, 3 to 5 minutes. Stir in the garlic and cook until fragrant, about 30 seconds. Stir in the water, quinoa and salt. Once the water starts bubbling, cover, reduce the heat to medium-low and cook until the quinoa is tender, 16 to 18 minutes.
Remove from the heat and let the quinoa sit, covered, until the liquid is fully absorbed, about 10 minutes. Transfer the quinoa to a large bowl and let it cool for at least 15 minutes.
Meanwhile, tear the bread into large pieces. Pulse them in a food processor about 10 times, or until coarsely ground. Add the beaten 1 egg plus 1 yolk and the lemon zest; pulse about 5 times, or just until the mixture comes together. Transfer to the bowl with the cooled quinoa and stir it in, along with the sun-dried tomatoes, lemon juice, spinach and cheese, until thoroughly combined.
To divide the mixture into 8 equal portions, use a spatula to first press it evenly into the bowl, then "cut" it into 8 wedges, scooping each one out as you work.) Pack each portion firmly to form a 1/2-inch-thick cake that's about 3 1/2 inches wide. Arrange them on the lined baking sheet as you work. Cover and refrigerate the cakes for at least 1 hour, and up to 24 hours.
Heat 1 tablespoon of the vegetable oil in a large nonstick skillet over medium-low heat. Gently lay in 4 of the cakes and cook until well-browned on the first sides, 5 to 7 minutes. Use a flexible spatula to gently turn them over; cook until golden brown on the second sides, 5 to 7 minutes.
Transfer them to a serving platter and tent with aluminum foil (to keep them warm). Return the skillet to medium-low heat and repeat with the remaining 1 tablespoon vegetable oil and remaining 4 cakes.
Serve with yogurt dolloped on top, or on the side.