Bring 1 cup water to a boil in a medium pot. Add the spinach and peas and stir.
Cover and reduce heat to medium-high and cook for 4-5 minutes or until spinach is just wilted. (When you add the frozen peas to the boiling water, the water will stop boiling. It will take a minute or two to get back up to boiling, then another minute or two to wilt the spinach. Keep an eye on it to avoid overcooking.)
Transfer mixture to a blender and blend very smooth, starting on low and working up to high.
Blend in any optional ingredients as desired.
Let cool and serve or store for future meals.
Notes
Store in the fridge in small airtight containers for 3-5 days.
To freeze: Portion into a silicone ice cube tray and freeze overnight. Transfer the frozen cubes to a freeze bag and seal, removing as much air as possible, and keep frozen for up to 6 months.
To make this thicker, remove 1/4-1/2 cup of the water from the pot after Step 2 so you puree with less water. Add just as much as you need to get the mixture to blend.
To use frozen spinach, replace the fresh spinach with 1 cup frozen spinach. Stir into the boiling water just until warmed through for about 1-2 minutes in Step 1. Skip Step 2 to avoid overcooking. Transfer the spinach and peas to the blender with a slotted spoon and add a little water at a time as needed to make the puree. (You likely won't need as much liquid since the spinach is already condensed.)
Add avocado, lemon juice, or olive oil to add more flavor and nutrients to this recipe if you'd like.
Mix half and half with Apple Puree or Pear Puree to make a baby food combination.
Sprinkle on a little grated Parmesan cheese for older babies to add flavor.
Stir leftovers into pasta or rice or quinoa as a simple sauce.