PWM to DC converter with op amp - slowly but steadily rising output voltage
Just tie the bottom end of the 10k resistor (on the inverting input) to 0V and see what happens - it should work just fine like this. Trying to create an offset with the two 1k resistors is missing the point of what this circuit is intended to do.

Also, you might struggle to get all the way up to +10V on the output with only a +12V supply.