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.
Step 3: The Settings
You can read the instruction manual of the RKC INSTRUMENT Rex C-100 PID controller.
Holding "set" you can change the parameters of the controller calculations. I list the values I used:
AL1 (not used): 0400
Holding "set" again it will save your values, and pressing "set" a single time it will let you change the temperature value for your oven (green value).
Step 4: The Match
The thermocoupler has to be inserted near the cooking surface, so to retrieve the right temperature, you see it in the picture embedded in the grate.