Melt 2 tablespoons butter with oil in heavy large pot over medium-low heat. Add onion and sauté until beginning to soften, about 6 minutes. Add garlic and sauté about 30 seconds. Increase heat to medium-high; add asparagus tips, sugar snap peas and thyme sprigs and sauté 2 minutes. Sprinkle with salt and pepper. Add vegetable broth and lemon juice, then lettuce and peas. Cover and cook until lettuce is wilted, stirring frequently, about 2 minutes. Whisk 1 tablespoon butter into ragout. Remove from heat.
Sprinkle scallops with salt and pepper. Melt remaining 2 tablespoons butter in heavy large skillet over medium-high heat. Working in batches, add scallops to skillet and sear until brown and just opaque in center, about 2 minutes per side. Divide ragout among 4 bowls (or three, if you eat like Jeff and I). Top with scallops, dividing equally. Garnish each with 1 lemon wedge.