Rinse dry blackeyed peas thoroughly.

Place beans in a saucepan and cover with a generous quantity of water.

Bring to a boil and cook, uncovered, until beans are tender, about 1 hour.

Meanwhile, heat butter in a heavy-bottomed stockpot over medium heat.

Add diced onions to the stockpot and cook until golden, about 5 minutes.

Add minced garlic and cook for 2 more minutes, stirring occasionally.

Add ground cinnamon, black pepper, cumin, coriander, and red pepper flakes to the stockpot.

Fry the spices gently until fragrant, about 1 minute.

Drain excess water from the cooked blackeyed peas.

Add the drained blackeyed peas to the stockpot, along with salt and chicken stock (or water).

Bring the soup to a simmer.

Reduce heat to low and cook gently, uncovered, until flavors are combined, about 20 minutes.

Add diced tomatoes, brown sugar, and minced oregano to the soup.

Cook for an additional 10 minutes, stirring occasionally.

Serve warm, garnished with plain yogurt.