1. To prepare filling, heat a large nonstick skillet over medium-high heat. Add 1 1/2 teaspoons oil to pan; swirl to coat. Add potato and next 6 ingredients (through black pepper); sauté 5 minutes. Add garlic; sauté 1 minute. Coat a 5-quart electric slow cooker with cooking spray. Transfer vegetable mixture to slow cooker.
2. Heat remaining 1 1/2 tablespoons oil in pan over medium-high heat. Add 2 1/2 tablespoons flour, stirring with a whisk. Cook 1 minute, whisking constantly. Gradually add milk and broth, stirring with a whisk. Cook over medium heat 3 minutes or until thick and bubbly, stirring constantly with whisk. Pour sauce into slow cooker. Stir in peas, thyme, and onions. Cover and cook on LOW for 3 1/2 hours or until vegetables are tender.
3. To make biscuit topping, weigh or lightly spoon 7.5 ounces flour into dry measuring cups; level with a knife. Combine flour, baking powder, and next 3 ingredients (through black pepper) in a large bowl, stirring with a whisk. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Stir in cheese and chives. Add buttermilk, stirring just until moist.
4. Increase slow cooker heat to HIGH. Drop biscuits onto filling in 8 equal mounds. Cover and cook on HIGH for 1 hour and 15 minutes or until biscuits are done. Uncover and let stand 5 minutes before serving.