In a skillet, heat the olive oil. Add the onion and cook until soft and golden. Add the chard stems and cook until softened, then add the chard leaves and garlic and cook for a few more minutes. Let cool for a few minutes and add the eggs, cheeses, ricotta, salt and pepper. Mix well.
Preheat the oven to 350F. Brush a 9- or 10-inch tart pan with a sheet of greaseproof paper. Brush the phyllo dough with butter and arrange in the pan. Repeat the procedure rotating the sheets allowing the excess to hang over the pan.
Pour the chard filling into the phyllo-lined pan, on top the green onion, lemon zest and thyme. Scrunch the overhanging phyllo around the edges of the pan to form a lip and brush it with butter. Bake for 35-40 minutes or until the outsides are golden and cooked through.
