How to cook fresh chicken soup on Shabbos morning

לז"נ ר' שרגא בן ר' אברהם צבי הלוי ע"ה

יום השנה ח"י אדר           

לא תבערו אש בכל משבתיכם                                                                     

    How to cook fresh chicken soup on Shabbos morning

Most* Poskim hold, Lighting a fire on Shabbos with a magnifying glass is אסור מדאורייתא. It's no different than creating a fire by striking a match.

1) One may not cook with תולדות חמה. e.g. Cooking an egg in a pot that was heated up by the sun. (egg heated by the pot)

2) One may cook with חמה e.g. Placing an egg under the sun. (egg heated directly from the sun) 

*According to Mori V'Rabi Harav Hagaon R. Shlomo Miller Shlita, One may place food under the sun when a magnifying glass

is directed on the food and not on the pot. Since the food is being cooked directly from the sun it's בישול בחמה which is

permissible. A magnifying glass just increases more sun.

*עיין כף החיים או"ח שי"ח:ג

**ספר שבת שלמה מאת ידידי הרה"ג ר' עזריאל יהודה געשטעטנער