Doomsday Preppers season 3 episode 8 showed a guy who took the Fresnel lens from an old television set, and with just it and some sunlight, was able to produce enough focused heat to make wood burst into flames.
According the Wikipedia article "applications include off-grid cooking, melting metals, generating solar steam and powering Stirling engines." It can also increase the amount of sunlight hitting solar cells.
-- On the episode, he had his Fresnel lens had photocells to sense where to move it to, so it moved with the sun. He put black rocks at the bottom of a glass jar, to absorb heat from the glass so it doesn't break, and the lid not screwed on too tight so volatile gasses will leak out instead of exploding.
Care must be given to not burn your skin, or do permanent damage to your eyes.
On the episode he proved you can melt zinc and forge it into knives.
Add in a parabolic mirror, focusing the sunlight up to a metal grill, and you can cook chicken without problems.