Heat 1 tablespoon (15 ml) oil in a non-stick skillet over medium-high heat. Cook the onion until soft, about 5 minutes. Add the garlic and cook 1 more minute. Set aside to cool.

Squeeze all the moisture out of the drained spinach using paper towels. It should be reduced to about the size of a tennis ball. Crumble spinach into a large bowl.

Add the onion mixture, feta, dill, mint, and lemon zest to the spinach. Mix to combine.

Break the ground chicken into large pieces, add to spinach mixture along with the egg. Stir to combine. Divide mixture into equal portions — 6 for large burgers or 12 for sliders – roll into balls without compressing. Gently flatten to between 3/4 inch and 1 inch (2 cm to 2.5 cm) for burgers and 1/2 inch to 3/4 inch (1.25 cm to 2 cm) for sliders. Using your thumb or a spoon, create an indentation in the centre of the burger. Refrigerate until ready to cook.

Heat 1 tablespoon (15 mL) oil in non-stick skillet over medium heat. Cook patties until internal temperature reaches 165°F (74°C ), about 5 minutes per side for large patties or 3 minutes a side for sliders, flipping once. Transfer to a platter and allow to rest 5 minutes before placing on bun and topping with cucumber slices and tzatziki.