First, make the dough. In the bowl of a stand mixer, add yeast, sugar and warm water. Mix with a fork to get all the yeast in the water and allow to sit for about 15 minutes until it gets all foamy and bubbly. That's when you know the yeast is activated.
Next, in another bowl, sift together the flour and salt. Add eggs, lemon zest and gently mix the eggs into the flour.
Add the flour and egg mixture to the yeast and use a dough hook to begin mixing everything together. You may have to use a spatula to push flour into the mixture, so check. Continue mixing for a few minutes until the dough comes together and starts to look smooth and forms a ball. Next, add butter, a few tablespoons at a time. If it's too sticky, add some more flour, up to 1/4 cup. It should look mostly smooth.
At this point, I like to knead the dough my hand to see how it feels. Turn the dough on a lightly floured surface and knead by hand for a few minutes. I wound up adding 1/4 cup more of flour, slowly until it looked smooth and slightly sticky. When done, form the dough into a ball and place in a lightly oiled bowl. Cover with a towel and place in a warm spot to double in volume.
While dough is rising, make the filling. Drizzle a bit of olive oil in a skillet and on medium heat saute onion until lightly browned, about 5-8 minutes. Then chop the rainbow chard into fine pieces and remove any large, hard stems. Add chopped chard and saute until wilted. Add raisins and season with salt and pepper. Saute for another minute until raisins plump up. Reserve to a bowl and add crumbled feta and toss to combine.
Now check on the dough and once it's doubled in volume, punch it down and turn it onto a lightly floured surface. It will feel a bit oily and that's ok. Roll it out to 1/4 inch thickness and use a round 2 inch cookie cutter and cut circles (or any glass will work too).
Take 1 of the circles and place about 1 Tb of filling in circle. Then top with another dough circle, crimping or folding the edges, making sure there are no holes and it is sealed very well. You can also use some water to help seal the edges. When done, place on a baking sheet.
Bring oil to about 360-365 degrees F. A candy thermometer is highly recommended! Once oil is hot enough, slowly place 2-3 sufganiyot in the hot oil. Fry one 1 side for about 2 minutes until golden brown and gently flip to the other side and fry for another 1-2 minutes until golden brown and puffed up. When done, place on a paper-towel lined baking sheet and sprinkle with salt. Serve immediately.