Direction :

1. Melt the butter in a large pot over medium-low heat. Add the onion, garlic, and crushed red pepper flakes (or less or none at all if serving to children). Season with a pinch of salt and pepper. Stir to combine, then cover pot and cook, stirring occasionally, until softened and just starting to take on color, 15 minutes.
2. Meanwhile, trim the bottoms of the broccoli stalks. If stalks are thick, peel the outer layer. Using a knife, separate the hefty stalks from the florets (the thin, little stalks attached to broccoli tops are fine to leave intact). Set aside florets. Coarsely chop the stalks. 
3. Peel the potato, and coarsely chop it.
4. Add broccoli stalks and potato to pot and stir to combine. Pour in 5 cups water, add 1 tablespoon kosher salt, and bring to a boil over high heat. Reduce heat to medium-low and keep at a simmer. Cover pot and cook until broccoli stalks and potatoes are completely tender (test with a paring knife), 20 to 25 minutes.
5. Meanwhile, chop reserved florets into small pieces. Add half of the florets to pot, cover, and cook until bright green, 3–5 minutes. Purée soup with an immersion blender or, if using a traditional blender: transfer soup in small batches to the blender, let cool for 5 minutes, and purée until smooth, taking care to remove the center lid from the cover, and to use a kitchen towel to cover and allow steam to escape (and avoid explosions) while you blend.
6. Return soup to medium heat and add remaining florets. Cover pot and cook until bright green, another 3–5 minutes. Add 2 cups cheddar cheese and stir until smooth. Reduce heat to low.
7. Remove soup from heat and stir in ½ cup plain Greek yogurt. Season generously with black pepper; taste for salt.
8. To make the croutons: In a large skillet over medium-high heat, heat a hefty splash of olive oil (2 to 3 tablespoons). Add the cubed bread and a good pinch of sea salt. Let cook, undisturbed, for one minute. Check one cube. If it’s beginning to turn golden, give the cubes a stir or flip each one over with tongs or forks. Cook for 3 to 5 minutes or until cubes are evenly golden and beginning to crisp. Pile into a bowl and serve alongside the soup.