Preheat a large pot or dutch oven over medium-high heat.
Add the sliced onions and salt to the pot. Saute, occasionally stirring for about 8 minutes (or until softened and slightly translucent).
Add the carrots and red pepper flakes (if using) and saute for another 4 minutes, stirring occasionally.
Add the broth to the pot and stir to deglaze the pot, and make sure nothing is stuck to the bottom.
Gently stir the kale under the veggies. It will seem like too much at first, but the kale wilts quickly. Do it in batches if needed, as soon as you have one batch in start with the next.
Once all the kale is in, cook over medium-high heat, stirring occasionally, for about 10 minutes or until the broth has reduced down to nearly nothing.
Pull the pot off the heat and add in the lemon juice. Give it a good stir and then taste for seasoning.
Serve either on a plate or in a bowl. You can top with more lemon juice if desired, or my favorite, a good splash of red wine vinegar.