Bring the water to a boil in a medium saucepan over high heat. Add the frozen baby lima beans, return to a simmer, and cook until just tender but not mushy, 8–12 minutes. Drain and set aside.
Heat the unsalted butter and olive oil in a large skillet over medium heat until the butter foams, about 1 minute.
Add the onion and red bell pepper; cook, stirring occasionally, until softened and translucent, 5–7 minutes.
Stir in the garlic and cook until fragrant, 30–60 seconds.
Add the fresh corn kernels and the cooked lima beans; season with the kosher salt and black pepper. Cook, stirring occasionally, until the corn is bright and tender-crisp and the flavors meld, 5–7 minutes.
Fold in the tomato and fresh thyme leaves; cook until the tomatoes just begin to soften and release some juices, 2–3 minutes. Remove from the heat.
Taste and adjust seasoning. Let stand 2 minutes to settle. Sprinkle with fresh parsley (for serving) and serve warm.