To a large skillet on medium heat, add chopped bacon and cook until crispy, 3-5 minutes.
Remove bacon and place on a paper towel lined plate.
Discard (or save for another use) all but approximately 1 Tablespoon of bacon drippings.
Add green beans to the pan and cook over medium heat until they are lightly browned, stirring occasionally (about 7-8 minutes). Add garlic and bacon. Toss and cook for 1 minute.
Meanwhile, in a dry small skillet on medium heat, toast sunflower seeds until lightly browned, about 3 minutes. Remove from the pan to cool or the seeds may burn (and turn bitter).
In a large bowl, toss together cooked beans, bacon, sunflower seeds, tomatoes, lemon juice, salt, and pepper.
Serve warm or cold.
Notes
Fresh green beans are best, but frozen or canned beans will work.