Soak the split peas overnight or at least 2 hours in warm water, drain and rinse. See notes for pressure cooker directions.
In a deep pan, add the oil. Once the oil is hot, add onions, ginger, garlic, and chopped Serrano or jalapeño, if using. Cook stirring occasionally, until translucent. Add the turmeric and berbere, if using, and mix well.
Add rinsed split peas, salt, pepper and water. Mix, cover and bring to a boil on medium heat.
Reduce heat to low and simmer, partially covered for 45 minutes to 1 hour, or until peas have softened to your desired consistency. Taste and adjust salt and spice if needed.
Mash the peas if desired. Add some lemon juice, if desired, and serve. Tastes best with a sour flat bread, like Injera, but you can also serve it with rice or flatbread of choice.