Place the Swiss chard in a large saucepan with water to cover, add salt, and bring to a boil over high heat.

Reduce the heat to medium and cook until tender, about 15 minutes.

Drain and chop finely.

Heat 1 tablespoon of olive oil in a medium skillet over medium heat.

Add the chopped onion and cook until wilted, about 5 minutes.

Add the Swiss chard, salt, black pepper, and oregano.

Cook until the chard wilts and absorbs the seasonings.

In another saucepan, heat the remaining 2 tablespoons of olive oil and butter over medium heat until the butter melts.

Add the flour and cook, stirring constantly, until the sauce is thickened and smooth, about 3 minutes.

Add the Swiss chard mixture and cook, stirring occasionally, for about 10 minutes.

Spread the mixture out on a dinner plate and let it cool.

Refrigerate until firm enough to handle (about 1 hour).

Shape the Swiss chard mixture into 1-inch balls.

Place the eggs and bread crumbs in separate shallow bowls.

Heat at least 1 inch of oil in a large skillet over medium-high heat (or use a deep fryer set at 365°F) until it turns a cube of bread light brown in 60 seconds.

Dip each croquette in the beaten egg, dredge in the bread crumbs, and place in the hot oil.

Cook, turning several times, until golden.

Transfer the croquettes with a slotted spoon to paper towels and let them drain.

Serve hot.