Combine garlic, cilantro, lettuce, onion, serrano, radish top, salt, pumpkin seeds, pine nuts, and tomatillos in a blender. Add 1/2 cup of the water and blend until as smooth as possible. There might still be some texture to the mixture. Leave contents in blender and set aside.
Heat the olive oil over high heat in a wide pan until it is smoking. Then add the contents of the blender and continue to cook over high heat, stirring constantly for five to seven minutes.
Add the remaining water to the pan, drop the flame to low, and allow to simmer for half an hour. Season with salt to taste.
Optional: If you prefer a very smooth texture, you can return the contents of the pan to the blender, working in batches, and blend until even more smooth.
The Pine Nut Pipián can be made ahead of time and keeps for five days in the refrigerator or freezer. After thawing, whisk or blend to restore consistency.