Set instant pot to sauté function and add your olive oil and fennel (and garlic if using), stirring occasionally, until fennel has softened and just beginning to brown. Add your celery and carrots and sauté for another 5 minutes.
Once done sautéing, hit cancel. Add your beans, broth, milk, oregano, salt. Stir to combine then add the chicken.
Seal instant pot and set to pressure cook for 15 minutes. When done cooking, manually release. Take chicken out and shred it with two forks.
Add chicken back to the instant pot with the flour and lemon or lime juice. Stir to combine and serve!