Thaw frozen spinach on the counter.
Slice the white and light green parts of the leeks into half moons. Discard the dark green tops or save for other recipes.
Rinse leeks twice in a colander or soak in water for 10 minutes and scoop out.
Cut broccoli into bite-size pieces with the tops removed from the stem and the stem cut into circles.
Zest and juice the lemon. Set aside.
Heat olive oil in a large skillet over medium heat.
Cook leeks until soft and translucent, stirring occasionally.
Add scallions and season with salt, pepper, and crushed red pepper (optional).
Add broccoli stems and cook for 6-7 minutes until soft.
Add thawed spinach and cook for 10 minutes, seasoning to taste.
Blend the skillet contents into a sauce, thinning with water if needed, with an immersion blender or regular blender.
Remove from heat, stir in lemon zest and juice.
Serve over your favorite dish (e.g., pasta) and enjoy!