Char 2 anaheim chili over broiler until blackened on all sides. Wrap in paper bag and let stand 10 mins to steam. Peel core and seed chli cut into 1/2 inch pieces, set aside.
Blanch tomatoes in sauce pan of boiling water for 10 seconds. Transfer to bowl of cold water using slotted spoon. Drain. Pull off skins using small sharp knife. Cut tomatoes crosswise in half. Squeeze tomatoes gently to remove seeds. Finely dice tomatoes and set aside.
Peel outer tough stems from lemongrass. Finely chop enough to make 1 tbs. (note. you can buy frozen chopped lemongrass in asian stores).
Whisk lemongrassh, fish sauce, lime juice, mint, honey. 1 1/2 tsp garlic, and serrano chili in medium bowl. Gradually whisk in 6 tbs olive oil. (can be prepared 6 hours ahead. Cover and refridgerate chilies, dressing and tomatoes separately.
Brush both sides of steaks with remaining oil. Rub pepper and remaining 2 tsp garlic over steak. Heat heavy large skillet over medium high heat. Add steaks to desired deegree of dongeness, about 2mins a side for rare. Let stand until cool.
Cut steaks diagonally into thin slices, transfer to large bowl. Pour 3 tbs dressing over steak, toss to coat. Combine anaheim chlies, tomatoes, lettuces and chopped green onions in another bowl. Toss with remiaining dressing. Divide salad between plates, arrange beef decoratively on top.