A better solution is to buy a digital temperature controller with a thermocouple, and connect it so it will regulate the oven in place of you.
NB: this PID controller support 250V at 3A, it means 750 watt. For any oven more powerful than that value an external 5A relay is needed.
Step 1: The tools
To avoid to cut the power cable of the oven you need to use a female socket, and connect it to the thermo controller with some wires. So you need a pair of screwdrivers and a wire stripper.
Step 2: The connections
Connect the K type thermocouple to the 9 and 10 pins, paying attention to the positive and ground wires.
We don't use the "alarm" funtion in this project.