Add the baby spinach and water to a blender or food processor, and blend at high speed until completely smooth.
Add the bread flour and salt to the bowl of a stand mixer fitted with a dough hook attachment (or a large regular mixing bowl) and whisk together to incorporate.
Turn the mixer on low speed, and gradually add the spinach mixture in two batches, giving the flour time to absorb the water with each addition. If doing this by hand, simply stir with your hands as you gradually add the liquid.
The mixture will eventually form a shaggy, green dough after 5 minutes of kneading. If the mixer fails to bring it all together, turn off the mixer and push the dough together with your hands.
Once the dough has formed a relatively cohesive ball (it will look lumpy), continue to knead with the mixer for 10 minutes or by hand for 15 minutes. Avoid the temptation to add additional water, as this will affect the texture of your noodles.
Cover the dough with an overturned bowl, and allow it to rest for 30 minutes. During this time, it will continue to absorb moisture and become more stretchy and workable.
After the dough has rested, knead it by hand for a few seconds to remove any air bubbles. Form into a ball and cut it in half.
Take each dough half, and roll into a thin sheet on a floured surface, about 2mm thick. Flour the surface thoroughly, flip over, and flour the other side as well. You can do this by hand with a rolling pin or with a pasta roller.
If hand-cutting, fold the dough so you have 4 layers. Slice the noodles with a sharp knife to your desired thickness. We decided to cut them about ⅛-inch thick. As you’re cutting the noodles, gently separate them out with your hands and toss them in flour so they don’t stick. You can also pass them through a pasta cutting attachment.
Bring a pot of water to a boil and cook the noodles for 1 to 3 minutes, depending on the thickness. Keep an eye on the noodles as they cook and taste them to determine when they’re cooked (there is a lot of variation depending on how thinly they were rolled and cut, so test in real time to determine when they’re done). Serve in soup or with sauce as desired, or use in pasta dishes!