Press Sauté and heat the vegetable oil in the inner pot of your electric pressure cooker. When it is shimmering, add the chopped onion and bell pepper and cook, stirring, for 4 minutes, or until the onion is slightly softened. Add the millet. Sprinkle with the salt, pepper, and oregano, then pour in the water and stock, stirring to be sure nothing is stuck to the bottom of the pot. Press Cancel.
Close and lock the lid, making sure the steam release handle is in the sealing position. Cook on high pressure for 9 minutes. When it is finished, release the pressure naturally for 8 minutes, then turn the steam release handle to venting, releasing any remaining steam. Unlock the lid and open it carefully.
Remove the lid, rake the grains with a fork, and transfer to a large bowl. Add the spinach to the bowl, stir it into the millet, and let the steam wilt the greens. Set aside to cool to room temperature, tossing occasionally with forks to help keep the millet from clumping. When cooled, stir in the cucumber, olives, tomatoes, red onion, and chicken.
In a small bowl, whisk together the olive oil, lemon juice, vinegar, salt, and pepper. Pour half the dressing over the salad, tossing to coat all the ingredients. Taste and adjust the seasonings; add the remaining dressing if desired and toss again. Sprinkle the parsley and feta over the top and serve.