Mix together flour, oregano, salt, and pepper in a bowl. Dust chicken slices with flour mixture and set aside.
Turn on a multi-functional pressure cooker (such as Instant Pot) and select Saute function. Add olive oil and allow to get hot. Add chicken and fry until nicely browned, about 4 minutes per side. Remove to a plate. Add butter and allow to melt. Stir in mushroom caps and onions; cook for 2 minutes. Mix in wine, water, sun-dried tomatoes, and bouillon cube. Bring to a simmer and turn off Saute mode.
Place chicken pieces on top of the sauce in the pot. Layer spinach on top of the chicken. Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 4 minutes. Allow 10 to 15 minutes for pressure to build.
Meanwhile, bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally, until tender yet firm to the bite, about 12 minutes. Drain.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid.
Open lid and remove chicken to a plate. Make a slurry by mixing water with potato starch. Mix slurry into the Marsala sauce. Set mode to Saute, and bring sauce to a simmer. Whisk until sauce is thickened then turn Saute mode off. Stir in half-and-half until sauce is golden and creamy.
Distribute spaghetti amongst 4 plates, ladle sauce on top of the noodles and place chicken onto pasta.