Heat butter and oil over medium-high heat in large saute pan. Add shallots; saute until golden brown and crisp, about 7 minutes. Transfer with slotted spoon to paper towel-lined plate; set aside.
In same pan, bring mushrooms, green beans, cream, lemon juice, and thyme to strong simmer; season to taste with salt and pepper. Reduce heat to low; cover and simmer, stirring occasionally, until beans are tender but still offer some resistance to the bite, 15-20 minutes.
Remove cover; simmer briskly to thicken cream if necessary. Adjust seasonings and transfer beans to large serving bowl. Garnish with fried shallots and serve immediately.