Cook the bacon and onions in the pressure cooker: Choose the “Sauté” setting and add the bacon to the pressure cooker. (If you are using a stovetop pressure cooker, use medium heat.)
Let the bacon cook until it has condensed some fat and started to brown- approx. 7 minutes. Add onions and sauté until softened and translucent- approx. another 3 minutes.
Add in the rest of the ingredients while stirring and pressure cook: Add the green beans, diced tomatoes and liquid, water, salt, pepper, cayenne, and thyme. Stir well so all of the green beans are coated with some of the cooking liquid.
Secure the lid on the pressure cooker. Make sure that the pressure regulator is set to the “Sealing” position. Cancel the “Sauté” setting on the pressure cooker, then select the “Manual” or “Pressure Cook” setting. Set the cooking time to 7 minutes at high pressure. (For stovetop pressure cookers, cook for 6 minutes at high pressure.)
It should take about 10 minutes for your pressure cooker to come up to pressure, and an additional the 7 minutes of actual cooking will begin.
Release the pressure: Do a quick pressure release by immediately moving the vent from “Sealing” to “Venting” (careful of the steam). It will take a few minutes for the pot to fully release the pressure.
Serve the green beans: Use a slotted spoon to lightly transfer the green beans to a serving dish. Scoop the tomatoes, bacon, and onions, and spoon them over the beans. Serve hot.