Heat olive oil in a 12-inch skillet over medium-high heat. Add shallot and saute 30 seconds.: You should hear a gentle shimmer as the olive oil warms, with faint ripples when the pan is ready. The oil must be hot enough to sizzle the shallot on contact but not smoking, because scorching will add bitterness. If the oil smokes, lower the heat and start over with fresh oil, as burned oil will ruin the dish. Use a pan with a wide surface so the beans get good contact and a bit of caramelization.
Add green beans, salt and pepper, and saute 4 to 6 minutes.: In this brief window the shallot will soften and release a sweet aroma, not brown deeply. You should smell a gentle sweetness rise up from the pan. Move quickly because shallot cooks fast; if it starts to darken too much, reduce heat and stir more often. Overcooked shallot becomes bitter, so aim for translucent edges rather than full color.
Pour in water, reduce heat to medium, cover and simmer until green beans are tender, about 2 to 4 minutes.: At this stage you want audible sizzling as the green beans meet the hot pan, signaling contact cooking that develops flavor. The beans will begin to blister and show flecks of brown while still looking glossy. Stir occasionally so they brown evenly. A common mistake is crowding the pan which steams rather than sautéing the beans, so keep them in a single layer when possible to encourage even color.
Add butter and garlic and saute uncovered 1 minute longer.: The brief steam will soften the beans to a tender crisp, and you will see moist steam escaping from under the lid. This step preserves the bright green color and gives a lovely tender bite. Avoid oversteaming because the beans will become limp and lose vibrancy; check early and test a piece for the texture you prefer.
Off heat toss in lemon juice and serve.: With the lid off, the sizzling will return and the butter will melt into a glossy coating. The scent of toasted garlic should be immediate and fragrant but not acrid. Adding the garlic late protects its bright, aromatic quality. Watch closely because garlic can go from golden to bitter in seconds; remove from heat if you notice any sharp burnt notes.
Off heat toss in lemon juice and serve: You should first feel the warmth of the pan settle, then the bright citrus aroma will bloom as you toss. The acid wakes up all the flavors and balances the richness of the butter . If the dish tastes flat, a touch more acid or a pinch of salt usually snaps it into balance. Serve immediately for best texture and color.