Use 2 tablespoons of the oil to brush both sides of the bread slices. Arrange on a baking sheet and sprinkle lightly with salt and pepper. Bake until golden, flipping halfway through, about 7 minutes. While the toasts are hot, rub both sides with the whole garlic clove and return the bread to the baking sheet.
Toast the pine nuts in a large skillet over medium heat, swirling, until lightly golden, about 3 minutes. Transfer to a medium bowl. Add the remaining 2 tablespoons olive, minced garlic and red pepper flakes to the skillet and cook, over medium heat and stirring frequently, until the garlic is soft, about 1 minute. Add the spinach, in batches, sprinkling with salt after each new addition, until wilted and tender. Transfer to the bowl with the pine nuts, add the lemon zest and toss well. Drain the mixture in a colander and then roughly chop it.
Divide the spinach mixture among the toasts and top each with some crumbled goat cheese. Bake just until cheese starts to melt, 3 to 5 minutes. Sprinkle with more red pepper flakes if using and serve warm.
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